I was at a party where this guy I have a crush on was at, but he hates me. He came up to me and said "hi" and I turned away from him. And he was like "you're not gonna say hi back?" and I said no and he said wow. Why did he do this?

Maybe just maybe he changed his mind and was making the first move here and you disssed him so now you may have just blown your chance with him. Things are never clear cut or black and white when it comes to love so one always has to have an open heart and mind at all times and be prepared for that special moment or love could be gone  and may not come back. Try talking to him if youre still interested in him and if hes willing.  Good luck