The problem with Luffy is that he's just too damn carefree.
He lives his life with no regrets. Great for him, but not so much for those who haven't quite reached that same conviction yet. He's always charging into everything headfirst; no consideration for others.
He's not self-centered, and he sure as hell ain't selfish- it just never occurs to him that others don't see the world the same way. If he lives, if he dies; it's all the same to him. Life is just one big adventure waiting to happen. Bumps, scrapes, bruises, broken bones, near-death experiences are all just part of the fun.
And yet, somehow it doesn't quite matter that he's not as considerate as he could be. It's not important that he doesn't make excuses for the weak, doesn't slow his pace for those who might fall behind. Because he'll protect his friends to the end, even if it means losing his life. That's the kind of guy he is. Sure, it's bang or bust with him, but without his friends, either and both are completely meaningless.
Yeah, Luffy's carefree, but he's one hell of a friend, too, and that's worth more than anything else.