2 Answers

People may have many different reasons to make others' lives miserable. For instance, some had experienced cruelty themselves so they do the same to others in order to deal with what they had gone through.
Cruelty is common among children, who sometimes feel they need to bully others in order to be more dominant or get approval by others.
Sometimes people may feel that others try to make their lives miserable when infect it's not true. For instance, parents may punish their children for educational or moral reasons, and not in order to make them miserable.

People pass on what comes their way, kind of like a copycat thing. You will see someone driving very fast down the road and passing others like crazy and then another person’s see’s that and starts to do the same thing.

Monkey see - Monkey do.