In my experience, I find that if a man disappears for good, it's because of one of two reasons. They are:
1. He did something wrong or hurtful
I've never met a man who liked facing me and admitting wrongdoing. Neither have my friends. I once explained it to a girl like this: A lot of men are all about pride and dignity. Women are too, but we know we can't survive that way. We might need to approach a stranger and ask her for a tampon. Men can't or won't do that. They would run across the street and buy the tampon themselves. But women are willing to show a little vulnerability, admit we're empty-handed, apologize, and ask for help.
There's a book that really drives home this point about pride/shame/ego/dignity and dudes, and it helped me to really see what can go on inside a boy's pinhead. It's Larry McMurtry's wonderful and short novel, The Last Picture Show. I read it a long time ago, so chime in if I've forgotten something. Anyway (spoiler alert), there's a boy character who wrongs a woman he has been seeing. She is waiting for him to stop by and explain himself, and she is totally ready to forgive him. They love each other. But he never comes back. He is so ashamed. His pride and his embarrassment of his behavior keeps him away. To a woman, that makes no sense. What a waste of a good relationship! Just say "oops, my bad" and we'll hug you and forgive you. We're pretty easy that way. But men just can't face you for some reason. Next time this happens, just imagine him asking another dude for a tampon. That’s not gonna happen, either.
2. He suddenly realizes he's headed toward a relationship
Girls plan ahead. What will I wear on this date? Will I go to second base? What should I say if he tries something? Should I shave my legs just in case? Will he ask me for a second date and should I say yes? Will I end up marrying him? Guys don't think ahead. He might have said something about the future ("Let's go to Santa Barbara one weekend"), but it really says how he was feeling in that moment—happy, road-trippy—and not about wanting you in his life for a future weekender. (I know, false advertising! And such a strange way to express your feelings!)
At some point, maybe even six months into dating you, he realizes he's committing himself, and he takes the easy way out—he disappears. He knows this is a bad thing. You should know he knows—and that will have to be enough, because you probably won't convince him to come back. He's not ready for the responsibility of a relationship. What he knows (rather conveniently and optimistically) is that you'll be fine because a worthier man will snatch you up soon enough. Possibly a man who could ask for a tampon—on your behalf, if you were stranded, and in a complete bind.