I... define "nice" for me, maybe? What are you thinking when you think the concept of nice, here? Because I've met quite a few people who almost everyone felt good about and who treated everyone they regularly interacted with very decently who were bigoted rat bastards. I'm actually one of the few people I've
met that (for some ungodly reason I can't seem to figure out) tend to get good responses from almost everyone, and that has a whole
hell of a lot more with being polite (and more accurately, civil*) than being
nice; sometimes the only right thing to do is politely tell the other party to go screw themselves. And don't ask me
how, but somehow even the cases where I've gone and done that ended up with better working, so to speak, relationships with those involved. Streak will break sooner or later, but it hasn't yet.
And... maybe "mean", too. There's a difference between, say,
unwarranted hostility and aggression (which
I'd probably call mean) and
warranted hostility and aggression. You can disagree without being mean, after all. I can't actually maintain a beyond facile interpersonal relationship (nevermind anything approaching romantic) with folks that indulge in the former with any regularity, but the latter exercised well tends to endure me to people.
Caveat to everything above being I am and have been largely asexual for my life to date, due largely to environmental factors, but I can probably chime in a bit for the very rare cases I've actually felt attraction toward people... providing I understand the question

*Which translates into something along the lines of very fundamental, underlying respect; to wit, my personal internalization of civility is the tact recognition that everyone I interact with can kill me, that I can do the same, and that we've decided -- perhaps implicitly, perhaps explicitly --
not to, and should both respect each other for that decision. Civilization, yeah.