That is such an open end question .. Hard to answer .. Given the premise that ALL guys are mean .. Which obviously is untrue.
Why are guys mean to girls when the girl hasn't done anything wrong?
They're not all mean and you shouldn't allow anyone to disrespect you. I take it you talking about one person not all guys. If a group of boys are bothering you and being hateful then you have to get someone to help you because this is a problem you can't take care of yourself.
It would be helpful if you could give more info.
Your being kinda steroetypical but for those who do fall under that title it's because they judge you once they see you . This is not only guys though.