I don't know much about your situation, but here are the "usual suspects," tell me if any sound familiar.
1) THings moved too quickly. The relationship started and everything was great, but maybe you got a little excited and got too far ahead of yourself, and he got spooked.
2) You spent a little too much time together. When you get a "high momentum," going, the "slow downs," seem much more devestiating.
3) He wasn't the most faithful person to begin with. But I won't point fingers at a person I won't know. ;)
4) You were great, but he found someone that was just so great he couldn't resist.
The reason he probably doesn't tell you the exact reason may because he himself isn't certain, or because he's just trying to avoid awkward situations.