Well, people will treat you the way you treat them. Unless they're jerks. Well, it's like this- if you're nice, you're not guarrenteed to have friends, and there may be people who don't like you. But if you're nasty, people definitely won't like you. People don't like being treated poorly.
And if you don't treat people right and you end up alone, you end up unfulfilled. I know someone who's selfish- always has to have her way, never admits when she's wrong, never does anything for anyone. And I don't think she even realizes she's like that- she's like a child, really. But she wonders why she can't make friends, and why she has problems with her family. It's sad, really.
Plus, I think hurting other people disconnects you from them somehow. You may not realize it at first, and it may not be something obvious, like getting the same treatment, but... I've head that it's impossible to hurt someone else without hurting yourself, because we're all part of the same whole. I believe that.