Luffy is generally a happy go lucky kind of guy. Cheerful, enjoys having fun and going on adventures. He's always looking for the "fun" or "adventure" out of a situation. While he rarely shows it, if an occasion arises where he needs to take charge and be serious, he absolutely can take the helm. In his canon, he's a leader after all, a pirate captain (and quite a few other things but I don't play him at that point). He's also very much a dependent type, not quite codependent but very much leaning towards it. He isn't capable of doing much else other than fighting. He'll need help with a lot of things, like cooking, cleaning, directions... He's not as bad as Zoro, though! He's perfectly capable of spending time alone but, again, while he won't say or show it, he does get lonely without a lot of friends around. He's had a very tough childhood (trauma, losing an important person, neglect, and unintentional child abuse) which shaped how he views things. He doesn't view it as bad, actually views most of it as things that brought him closer to his family, but all of this has culminated in an "if I die, I die" kind of mindset.
In regards to his childhood. Luffy didn't meet his father until he was a teenager and didn't even know the man helping him was his father at the time. Growing up, he was raised in Windmill Village, mostly hanging around the bar with the bartender, Makino, and listening to the visiting pirate crew of Shanks. When he was seven, he ate a cursed devil fruit and shortly after, ended up fighting with his idol, Shanks, over how he handled a fight with some mountain bandits. Luffy thought he should just fight whenever he was insulted but Shanks said that as long as it wasn't "serious", it didn't matter if he was insulted or even if others spit on him. A few days later, Luffy ended up picking a fight with the same mountain bandits and Shanks came to his rescue, stating that while he didn't mind being personally insulted, no one better mess with his friends. This is a lesson that stuck with Luffy for life. Personal insults won't bother him but anything that involves hurting his friends or making them cry is considered an insult to him. He's shown several times in later events, saving others after they end up breaking down or get beaten up, stating that they're his friends. There's even a scene that mirrors that scene in the bar, where he refuses to fight a group of pirates in Java (an island) stating that it's pointless. He isn't looking to argue with them about ideals or change their mind. When they threaten to beat him up, he simply tells his first mate, Zoro, not to fight.
That isn't the only thing that carried over from his childhood. After Shanks left, his Grandfather came to town and moved him to a different area, where he forced Luffy to live with a group of mountain bandits lead by a woman named Dadan. Luffy did eventually warm up to them but would claim even as they raised him that he didn't like mountain bandits and this continued until he left to adventure. However, living with them and being trained by his Grandfather whenever he stopped by is what led to his resilience. As a child, it didn't matter that he was seven, he was told that he was expected to be self-sufficient. That included being expected to go into the forest and figuring out how to hunt his own food. This is also where he would meet his future adopted brothers, Ace and Sabo. Both are three years older then him and both were already self-sufficient, having basically raised themselves.
Luffy and Ace originally had a contentious relationship but Luffy quickly switched to trying to follow Ace around. He later stated that this is because that he didn't care if he beat him or hated him, being alone and lonely was more painful. However, their relationship didn't strengthen until Luffy was kidnapped at one point and beaten until his voice gave out from screaming and his face was bloody and swollen. Even still, he refused to give any information about Ace, because he didn't want to betray him. However, because he has a terrible porker face, it was obvious when he was lying about not knowing anything. (He still has a terrible poker face as an adult.)
After being rescued from this incident, he made a pack with Ace and Sabo to be brothers for life. This is a bond that should, and did for a time, strengthen the bonds between the young boys. However, because one of them was the son of a noble, Sabo ended up having to leave them and go back to living with his family, whom he hated. Unfortunately, after plot events that would take too long to get into, he found out about a plot to set fire to the entire area and kill everyone living outside the noble city, including his brothers. He tried to save them but wasn't able to warn them in time. He didn't know if they survived but chose to believe they did. This led to him needing to leave, however, and the first day he set out to sea, his entire ship was blown away with canons, seemingly killing him. A witness to this incident led to the boys finding out and them crying over it. Between the two of them, they promised that they both would live freer than anyone else, in order to make it up to the brother they lost. And that they would fulfill their dreams, since Sabo couldn't fulfill his.
Luffy has a habit throughout the series of deciding something and then not being willing to hear someone else out, even if it involves them. It happens as early on as the third chapter, where he meets Zoro and decides he should be part of his crew, even though Zoro refuses. From that point on, everything Luffy does is to convince Zoro to join his crew. He does this again with Nami and Sanji, deciding for both of them that they were going to be part of his crew, despite their initial objections. Sanji even takes the time to explain the reason why he can't join and Luffy instantly rejects his reasons, saying he'll be part of his crew no matter what.
In this way, Luffy is selfish, self-centered, stubborn, and a bit egotistical. However, despite often times demanding this, if someone actually refuses, he won't force them. He tends to have a very high EQ and intuition, so he won't simply accept no for a half-assed reason. He won't back down and will continue to press the issue, disregarding how this might genuinely upset and even anger the people he wants to join them. While it isn't done out of maliciousness, he very much is demanding to have his way. His childishness, too, while often framed as endearing to those who know him, takes some time to get used to. He doesn't exactly throw tantrums but he's been known to pout over not getting his way. He expects to be able to barrel over others, getting them to bend to his will. He hates sharing food and can be rather annoying at times, which can easily grate on other's nerves. Someone who isn't good with compromise is bound to have a lot of issues.
He also has zero tact or ability to read a room. While it can be framed as a positive at times, many times, it leads to him ending up in more trouble. He says exactly what he's thinking and does what he wants, without thought for the consequences. One example of this is during a scene in Alabasta, where he's helping Vivi, a princess, try to stop a civil war in her country. During their journey, at one point he simply stops and says he won't progress any further. When pressed on why, he says he doesn't want to. He wants to fight the main bad guy. Even with reasoning on why they shouldn't do that and need to focus on the task at time, he refuses again. In a moment where he callously speaks the truth, he tells her that she's naive for expecting to be able to stop a war without anyone dying. That people will die, no matter what she does. While the story frames it overall in a positive, the actual moment is almost cruel. She's near her breaking point, trying to stop everything that's happening and he tells her it's all basically pointless. Yes, things work out for the better but there were kinder ways to put it. Luffy isn't the type of person to try to nicely frame these words, however. He is blunt, to the point, and if he hurts your feelings while doing it, he's sorry but he'll say what he thinks needs to be said. He's very much the rip the bandaide type of person, regardless of how much it might hurt the other person.
Another big negative that comes up is his lack of self-preservation. There are many scenes where he states that he's chasing his dream and is fine dying in order to do it. He pushes his body to it's limits and beyond in Ennis Lobby, to the point where he literally can't move after fighting, not even to save himself. There's a scene even earlier, while he's on the Baratie, the ship where he first met Sanji. During that fight, despite knowing that falling into the ocean would mean he would instantly be unable to move and would drown, he fought all out on only small boards and planks and eventually won the battle--only to fall into the ocean, unable to swim and unable to save himself. There are too many times to list where he pushes his body to it's absolute limit, until he's near death, but he views it as necessary as long as he can help his friends.
On that same negative trait, during Thriller Bark it's shown that Luffy also might be terrible at lying but he's a master at hiding pain. He never shows or lets on the amount of pain he's in after a battle, just laughing and smiling it off. However, he'll usually sleep for two to three days afterwards, showing that his body needs serious healing. And at the end of the fighting this time around, in order to spare the life of his captain, Zoro agrees to take on all his pain. When given even a small portion of it, he's left screaming and writhing in pain. The amount of pain Luffy puts himself through is to an unimaginable degree and he doesn't let anyone know. Personally, while I think it's admirable that he wants to protect his crew from even knowing the level of his pain, I also find this to be an incredibly negative trait. In the same arc, Chopper comments on how pain is the body's way of letting you know that you need to stop before you get to a critical point. In a similar way, personally, I view his inability to share that way as a flaw of him not being able to share emotional pain and vulnerability even with those closest with him.
A final negative trait that I would like to point out is him living constantly in the present. While the story paints this admirably, I find some flaws in this when it comes to Luffy. By living in the present, it leads him not sharing his past even with his crew, who arguably are his closest friends and even like his family. He doesn't mention anything about his brother Ace to them until they actually meet Ace, nor does he tell them about his Grandfather until they meet his Grandfather. He briefly mentions Shanks, who turns out to be one of the most powerful figures in his world, but he doesn't really expand on his relationship with him. Despite sharing so much of his life, happy and sad moments, he doesn't open up about his past at all with them. And even once they meet two of these important figures in his life, who literally shaped his childhood and had a hand in making him the person who he is, he only briefly explains his relation to them and doesn't go into details.
In the same way he doesn't dwell in the past, he also doesn't seem to take the future into consideration. There are more than a couple incidents where him not thinking through the consequences leads to problems in the future. Just starting with chapter two, for example, he had no plan when he set sail out to sea. He didn't really have enough food, he had no idea where he was going, he couldn't navigate, and not long after leaving, he gets caught in a whirlpool and almost dies. Further into the future, during the Arlong arc, he wants to try out a "cool new move" that he "just thought of" and slams his feet into the ground, getting himself stuck in place. Again, because of his carelessness and lack of planning or forethought, he ends up almost dying from being unable to pull himself free. He slowly gets better it as time goes on but at this point in his canon, he would still be struggling heavily with his planning and strategies.
However, despite all his selfish antics, he's incredibly selfless when it comes to protecting his loved ones. He'll break his body apart and literally kill himself if that's what it takes to protect his friends. He learned many of his abilities in his canon world, despite most them having incredibly serious and dangerous side effects to his body, in order to protect his crew. He will continue to use them, despite the negative side effects sometimes leaving him in vulnerable positions, simply to protect others. He doesn't mind sacrificing himself for people he's recently met, either. As of this point in canon, he's known his entire crew for less than a year. (Approximately 9 to 11 months.)
In many ways, Luffy is a contradiction but that's what makes him so human. He's kind, yet selfish. Selfless and protective of his friends, while being self-serving, always wanting his own way. Fun loving, yet able to become serious on a dime. Always looking for an adventure, yet recognizes that the world is a dangerous place, where things could go wrong at any moment. His main thing is to be in the present. Worrying about the future or focusing on the past won't do him any good. It should be noted, however, that if he ever falls into a negative cycle, it'll be nearly impossible for him to get out of it on his own. He'll continuously beat himself up on things he should have done differently, despair, and will need help from others, a reminder of everything he still has to live for. He's one break down away from being catatonic. Fortunately, or unfortunately, he doesn't break down often.