I dont understand this. Why bully eachother?

In my small high school i see kids gettin made fun of and pushed around all the time!!! I have never bullied anyone, or pushed people around!! I have always made friends by the personality not by the looks!! I get called a nerd cause im in Band, ALL the time, but i stick up for myself, i dont care because i love to play music, and not one single judgement will change that!!! I have seen my bestfriend get slapped in the face by a girl, and pushed around by that girls friend!! And i was the first one to defend her, cause she was to scared to defend herself!!! I have faught and been beat up for defending people who get pushed around!!! It hurts me to see it, Not every is perfect, and nobody is the same!!!! Wy beat people up for it?