People bully other people for a lot of reasons, the most common reason being that they have poor self-esteem and are (or were) themselves bullied at home. People turn out badly because they have been treated badly. The boy who calls people fat? Isn’t it funny how he is chubby himself? Guess what–at home, someone is really cruel to him about being chubby, and he is taking it out on you. That average-looking girl who calls lots of other girls ugly? She is insecure about her looks.