Why do men disappear when it's just getting good?

I would be prepared to move on. You should try to contact him, but only try every so often. Otherwise he might look at it as nagging. If you have a way to leave a message that he'll get, do that. Just let him know that you wish things had worked out and that you just want to know if things are over or not. Try to make it sound like it's not a big deal if things are and that you just want clarification. It might be enough to snap him out of his mood if he hears that you wished things had worked out and yet you were still interested. It's sort of a "You're still there but about to leave" setting and forces him to either speak up or let it go. However, I would be ready for letting him go. After that, maybe 2 months or so, the friendship might pick back up.

Well, I would guess that there are two possible reasons- either he lost interest/didn't enjoy the first date (depending on how long the relationship was), or he's worried about getting close and so he has turned around and sacrificed what he had. Now, the first is easy to explain (and women do this a great deal too) - it's easier just to disappear rather than have to say it's over and deal with the hurt feelings, arguing, the other person refusing to give up and so on.

The second one happens to some guys and it's hard to explain. It has to do more with self image than anything else I think. View it like this, thoughts running through the mind of a guy: "She seemed upset last time (could really have been nothing). I bet she's mad at me, but I can't even figure out why. Well, that figures, I never seem to be able to keep up with her. Why does she bother with me anyway? I'm (list of bad traits) and she's (list of good traits). She could do better than me." Now, here we have the break up coming and we go back to the point where it's better just to leave than fight or hurt each other with words. He might not want it to end at all, but a bad self image and a his misunderstanding how she feels makes him think it's for the best. I'm not sure if that helps any and it's possible I'm way off base. I just know these two events happen and they could be the cause.