Why do people feel the need to put others down? Please help me figure this out?

A lot of people i know seem to call others a "scrubber, sl*g, sk*t, tart" etc.. Why do people do this? Do they get thrills out of it? I get put down a lot but i don't know why people need to try and lower my self esteem, if i wasn't as strong as i am i feel i'd be pretty much depressed. Whats the point in making people feeling bad?