Humans are selfish, selfish creatures. That's all you need to know. They go about blindly trying to fulfill their desires, and in the process hurt their fellow man in little or large ways, especially if they can do it with little risk.
Unfortunately, one of those selfish desires is to share (ironic, no?) their misery with everyone else, little or large.
Bottomline is, we're just monkeys that grew a brain. Go to a zoo with monkeys, and throw a banana to a not dominant male. More likely than not, the dominant male will kick the snot out and take the banana. The punked monkey will then go and beat on a less dominant monkey, and so on. This is humanity in a nutshell. Watch the daily interaction at work, at school and at home, and you will eventually see the pattern.