Gimme gimme gimme

I've been reading this biography of Louisa May Alcott the past couple days and it seems like the ongoing theme is "I should do this...but I did this instead." Even in her childhood diaries she writes stuff like, "I have to try harder to be nice, even though I really want to hit my sister in the face." It's making me think a lot about human nature and the nature we're born with. You get rewarded for doing nice things when you're a little kid - you share candy with your siblings, your parents say aren't you sweet; if you hoard all your candy, your parents ask why can't you be generous like your sister? But maybe it's easy for one child to share candy because it's in her nature and maybe the other child really has to force herself not to keep every Starburst she gets her hands on. Louisa May Alcott was more difficult than most children and definitely more difficult than her sisters and her parents gave her a ton of shit about it. And then she died of mercury poisoning.

Compassion, selflessness, bravery, strength of character, generosity, those are all "societal values." But maybe some people can't be compassionate, or selfless, or brave. I've been told that I should be nicer, and sometimes I try it for a couple days (hours), and it really doesn't go well. All the meanness builds up until I do or say something truly awful. Is it a lack of self-control on my part? Or are some people actually incapable of being nice?