Despite what you may think, many boys are only mean to girls when they have a crush on the girl in question. Have you ever noticed that a boy can appear to be really nice when it's just you and him, but appears boisterous and tries to be masculine with he's with his friends? It's all about street cred, and it's clear that many boys don't want their mates to see them as soft, warm and gooey inside. It just isn't cool.
Of course, boys are also quite mischievous by nature. Whereas girls are made of sugar and spice and all things nice, the other sex can be icky and full of horrid things. After all, who would want to go and play football all day?