I'm so sorry!
All any of us can do is speculate on that, but it's not necessarily a YM thing. Guys do that at any age. Apparently he wasn't ready to be a father, or commit, or whatever.
And it's not just guys. Women do that, too. Especially if the relationship isn't really a relationship (like a fling, friends w/ benefits, etc), or if she realizes that she doesn't want to be "tied" to this guy. I've known women to up and take off when they've learned they are pregnant, and don't even tell the father to be. Or they go get an abortion without letting the guy know they were even pregnant. Had my 23 year old guy friend's girlfriend do that to him. He found out about a month after the fact and he was very upset. He would have totally stepped in and raised the child if she didn't want it...but that's a whole nother topic.