Girl's Behavior

Why does this girl who rejected me now wants me?

Ok where do I start? I liked this girl, or so I thought so. I'm those straight forward kind of guy and I told her I liked her and if she wanted to hang out. She flat out rejected me and with the generic excuse. It's me not you. So meh I lost interest in her within an hout of getting rejected. Pretty fast to move on right? Apparently she told her friends that I'm a weirdo and other stuff insulting me behind me back. That didn't really affect me, since those are just words. I pretty much gave her the same amount of attention but to a lesser degree. 2 months later I met another girl, who not only was more beautiful than girl A but she had a better personality. And girl B liked me back. So we started dating. We had so much with each other and still are. Girl A notices how much fun Girl B is having with me and she realized she fucked up rejecting me. She even told girl b lies about me in order to break us up. I confronted Girl A about this, she pushed me to a wall and asked, "why don't you come back to me." I brushed her off and told her Girl B is the only girl for me. You had your chance and you messed up. Good thing girl b was watching us and found out about girl a's true colors.
So my question is. Why do girls reject guys than when they realize that they messed up try to get the guy back when he is happily dating someone else?
Why does this girl who rejected me now wants me?
Post Opinion