Why are some people mean?

I am a quiet person I stay inside a lot of the times when I was in high school I really didn't have friends to be honest with you I don't want any. I feel like some people that have been mean to people all their life need help. Yes, I am quiet and yes I do love keeping to myself it's nothing wrong that. I just want people to stop talking about me I really don't know how to talk without crying I been like that all my life!