Guys, why do you suddenly lose interest in a girl?

A lot of girls have been there. The guy they're seeing or are in a relationship with, suddenly loses interest and cuts it off. Some girls pick up on it right away, some don't see it and end up being treated badly because the guy just doesn't have the guts to call it quits.

So what are some of the reasons that make you "suddenly" not into her? Why the behavior change? I've heard some guys say that they "care" but they treat the girl like crap in the end. Is it conflicting emotions, or trying to push her away to make it easier or what?

Share examples! I'm sure some girls around here could use the insight.