Why do people make fun of others?

Like i honestly don't understand what people get out of it. At a baseball camp im in there's about 40 kids. most of them are good. but there are a few that are terrible. Im trying to help this one kid and make them better, but others make fun of him. he has a lisp and he gets made fun of for that. Kids are throwing as hard as they can at him and just downright dissing him. I try to stick up for him but they still continue to do it. His family paid the 60$, and he is learning and trying to get better. i just don't understand why some kids need to pick on him just because he isn't the greatest baseball player alive. The only reason it bothers me is because I love baseball, and i love seeing new kids try to learn the game. Im not conceited but i am probably one of the best at the camp and to see kids who aren't amazing make fun of him, really gets me mad. What else can i do to make sure no one makes fun of him and others at this camp?