Bullying, why?

I just got off of the phone with the principal at my daughters elementary school,  He called to let us know that she was a victim of bullying.  5 boys were picking on her because of something she has no control over.  She has psoriasis, a skin disease where her body produces extra skin cells.  It has never bothered her before, but now I fear she will become more self-conscious.  I am so sickened to think that she was treated in such a way by her peers.  Why do people feel the need to bully others?  Perhaps it is an age old question.  I remember being a victim myself and I have to say I don't understand it anymore today than I did when I was younger.  What happened to raising our children to love one another and accept their differences?