The number one reason a guy dumps the girl of his dreams is that he is afraid of settling down, afraid of a life change, afraid for reasons he may not even be able to enumerate if asked. Men are funny creatures. Often, they are as sensitive as women, but it is ingrained into many of them from a young age that their place in the social order is as the breadwinner, the soldier, the general tough guy. For some men, it is much easier to live up to that role without any kind of feelings messing everything up for him.
He's Not Ready to Commit
Whereas many women decide to settle down when they meet the right guy, most men operate on a timeline. They decide to settle down when an appropriate woman comes along at the right time. You might be the girl of his dreams, but if he hasn't quite decided to settle down yet, he might pass you by because you didn't come along at the appropriate time.
He's Fixated on the Worst-Case Scenario
There's a sexist old axiom about being able to tell whether a woman will get fat by looking at her mother. While this is a somewhat vile little "fact" that men bandy about in locker rooms and other all-male spaces, many men, intentionally or otherwise, take it seriously. If he gets the germ of doubt into his head, he will concentrate on the worst things about you -- your mother, your hometown and so on -- and ruminate on how you might only get worse.
He Wants to Play the Field
Related to their desire for the right time for commitment over the right person, many men are terrified of giving up their right to "play the field" or date many women. They may subscribe to the "grass is always greener on the other side" scenario, or they may simply be ladies' men. Sometimes, there is just nothing you can do about any of it.