Sometimes I think, as Gunman says, it's to do with their own problems and they just take it out on others because they're not really thinking properly or maybe they think that if they're not happy, then nobody else is entitled to be.
Other times I think it can be about genuine ignorance and arrogance. People like to impress others. Sometimes they'll think the way to do that is to be mean to somebody else - they'll think that if they do that, then it will make them look powerful/clever/funny and most importantly - somebody who's good side you'll want to stay on.
I'm agreeing with this. In school, there were some people that did seem to have anger-related issues, and they took it out on their own friends almost as often as they did on us unpopular folk. Not that
we ever received any apologies, but I think it's fair to say that they couldn't really help it sometimes, and if we were there, we were just easy to take it out on.
The majority of them definitely knew what they were doing, though, and it was pretty clear that they did it to impress their friends, knowing that they wouldn't get a reaction out of us. This one time, one of these popular kids must have been getting a little desperate for attention, so he beat up one of my friends for no reason at all, making up some story involving my friend saying something about his mum. I will never, ever forget his face when he found out that that friend's dad is a policeman, but anyway... I don't think there's any reason that this sort are mean aside from wanting to amuse their friends at our expense, but from my experience they definitely know how bad they're making the rest of us feel.