I've had to really work on this. I used to think that if I was kind to someone they would be naturally kind back and when they weren't it hurt. This mostly happened with girls that I wanted to be friends with...now I'm still nice, but super-picky with who I choose to let get that close.
Showing my true, real self is hard. Bullied, teased, made fun of for who you really were is hard to come from. That's why people put up walls, barriers, masks, to hide behind. They're afraid that people will do it again, just like the people before.