Jellybean -
It is REALLY odd that you had no sign it was coming, that he didn't talk to you about things he wanted to be different or issues he had or anything like that. You would think he would have given hints that he was unhappy.
If you have asked him what was wrong and he can't even tell you, then that there is even more odd. He should at least be able to give you some ideas of things you could work on (from his point of view).
I know this is hard to think about, but things couldn't have been that great if he felt this drastic step was required. He must have been unhappy but unwilling to talk about it for whatever reason. So you could say that the biggest issue in the relationship is that you guys weren't communicating regularly. You had gotten to the point that something huge and serious could be in the middle and it wasn't talked about at all.
So if nothing else, you could make that something to work on going forward - finding ways to communicate openly and healthily with your partner, so you always have a sense of how they are feeling and what you could be doing together to make you both even more happy.
