Glass Houses

I'm sure you have heard the saying People who live in glass houses shouldn't throw stone. Why is it that people feel the need to bring others down just to make them feel better about theirself? Why do we say such mean things to one another? I don't think I will ever truely understand. It will go on for the rest of our lives. I know adults that do it too. We get some kinda of satisfation from making other people feel bad. Its cruel.