Girls don't have courtesy in them. So when they are not in love they could be real mean.
Did you see the whole video? The song is not about the girl being cruel. Its about the guy not appreciating her and hurting her so much she had no choice but to leave. Once she was gone, he realized what he had. But by that time, it was too late. She had already found someone who didnt make her cry anymore.
Why must we hurt people and allow them to walk away to realize how important they are to us? Why cant we appreciate them while we have them?