What am I supposed to do when it's all a lie?

I don't understand why guys flirt and act like they're interested, and then act so different the next time you see them and they ignore you. This happened to me and I'm so hurt and angry. I felt like a guy kept staring at me (not in a creepy way, in a cute way) and my friend kept saying she thought he liked me.. and he kept saying he wanted to get together with me. Now he doesn't even talk to me on msn. I don't understand why. I'm hot (not to brag or anything, but I'm attractive lol) and smart. I just don't get it. But I guess if guys don't act interested fuck 'em and move on (fuck 'em in a figurative sense of course)...