Some people just enjoy being nasty, to see others feeling hurt or bad. And these people are often insecure, so gain a sense of power by putting others down and being unpredictable and hurtful. I dont believe happy, fulfilled people bully, Ithink insecure unhappy people do. They seem strong, but they are not. A strong person has no need to belittle anyone else. Its often hard to stand up to bullies, because they are good at mind games, but in the event you are dealing with one, dont get too close to them or too involved socially. They are often at their worst when they are alone with you, so dont give them the chance for a one on one. As long as you see them for what they really are (weak and pathetic) you will learn to treat them like water off a ducks back. As long as they know they have you under their thumb, they will keep playing their game.
Sometimes it can just be that maybe the bully has been through a hard time and has trouble expressing how she / he feels and tends to like to take it out on others. Another possibility is that they can just be plan mean and like to think that they are better.