Typically, it's their own insecurities talking. Knocking someone else down, somehow boosts them up in their own head.
Regardless of who the target may be, whether it's you or someone you know, don't let other people knock you down. Celebrate the things that are great about you! And quietly, feel sorry for the bully. They're not as secure as they might come off.
because people don't feel self confident about themselves so they make fun of and judge others so the can feel good about themselves. its really stupid really. but they do it because they don't feel good about their own selves.