Growing up is hard. Children are generally awful to each other. The world is filled with unhappy kids taking out their unhappiness on even less happy kids who then take that unhappiness out on still less happy kids. This cycle is often punctuated by tragedy.
People do this at every age, obviously, but one of the best parts of becoming an adult is realizing the shallow sophistry of bullying itself—that it has nothing to do with the bullied and everything to do with the bully's sick psychology. But when you're a kid and you already feel like you are alone and someone who appears to be popular and well-liked says something cruel to you, it can be hard not to think that they just may well have a point.
If time machines existed we could go and warn ourselves. "Look, young me, kids are going to say mean things to you but only because they're from a broken home and their father didn't go to their baseball game and they're beginning to suspect that maybe they aren't very bright and they have very little self-worth and they're trying to make themselves feel better about their own mediocrity by putting you in a position that allows them to think 'well at least I don't have it as bad as him!'" Then—poof!—we'd vanish in a puff of smoke and our young selves' would ride off to grade school with armor optimized for adolescence.