Why is high school so cruel?

I understand I am very weird and awkward compared to most people. I have eccentric tastes, don't have too many friends, and don't think like most people. I don't see how that is a license for people to constantly make fun of me both in my presence and behind my back though. I'm made fun of by both guys and girls alike for my various eccentricities. I try not to let it get to me, because I realize it shouldn't be worth my time, but it still makes me hurt inside. This is compounded by the fact that I have depression (been something I've had for over a year and a half), and I was recently dumped by my girlfriend for another person. Sometimes, when I'm alone and just thinking, I get such a strong emotion (usually hate and anger) that I feel like self-harming. I just clutch my arms not to let my hands do anything. Why are people so cruel in the time of high school? Why do people feel the need to **** on others for being different?