It's hard to be nice when you feel that the others are exploiting you in some way. Even if you can empathize with their situation, sometimes you cannot sympathize enough to put their well being over your own (mainly in cases where the absolute difference between the relative importance of their needs to your own, in your opinion as per your beliefs, is small).
Well, I'm an antisocial shut-in so maybe this is a fringe opinion to have for our techno-monkey tribes. :))
Being nice doesn’t mean putting their well being over yours. You can still set boundaries around what you’re willing to do for someone without being an ass about it.
And bad to act like an asshole.
If you act nice, and don't care what people think about you, you can't lose, and most people will like to have you around.
Assholes might still annoy you, but when that happens, your friends will help you to feel better.