Why do girls lose interest in me?

I've been in 2 relationships in the past 2 months. The first relationship was with a girl who basically chased me for a month and I finally decided to ask her out even though I knew she would possibly be moving hours away in a few months. After a week she decides to tell me that she is stressed out about her parents pressuring her to choose a college and so she tells them she is going to a school 4 hours away. When she tells me this I start to think, "What is going to happen to us?". I decide to think about it a bit more before I made any decisions, because she was a very good person and I saw a lot in her that I wanted in a partner. As a few days pass she begins to act more and more distant, and I react by just trying to be around her more and see if maybe she is just worried about moving. Turns out she was worried about it. 2 weeks later she tells me she doesn't want to waste my time because she is going to be moving anyways. She didn't even give me a chance to tell her how I felt about it. She rushes out of the building we were in and doesn't even look back. So I go home and think about what just happened. I felt like I needed to tell her how I felt about the whole thing. I sent her an email telling her that I think she is an amazing person and I don't want to lose her, even if that means we just stay friends until she figures out if she is really moving.(she said that there was a chance she might not move. I believe her exact words were "But if my plans change then maybe we can try again.") So I send the email and never get a response. I have a class with her the next day and she says noting to me about it. She waits for me to walk out of class with her, but just makes little small talk about the class. After noticing that she must not be telling me the truth and was just being nice to spare my feelings I just don't make any attempt really to talk to her anymore. So a few weeks later I find out that she is already dating another guy. On facebook of all places. She made it so I couldn't see her relationship status on facebook, but my friend could and he told me about it. I immediately text her, because I was too frustrated to talk to her, and told her she shouldn't have lied to me and that I guess I trusted her too much for her to actual be honest with me. It was very nice and sarcastic. So I talk to her the next day and she says why did you send me a creepy text? I was like "How was it creepy? You straight up lied to my face. And then you start dating another guy within a week?" She basically made it my fault for not telling her that I wanted her to stay and not begging her to change her life to suit mine. After talking for about 30 minutes and telling each other how we felt about the whole thing, she tells me she is in love with this guy and has only known him for 3 weeks and wants to spend the rest of her life with him. She also decided to not move away and stay because of him. What is she thinking? And why did she do it?