I'm not typing out the whole thing (yet, anyway) because I already did that once, but tl;dr: He felt like I wasn't being emotionally satisfied/was happy not to be single anymore/didn't care about him in the same way.
And if you saw me go through that box of tissues, you'd know how not true this was.
But before I contact him again for friendship purposes, the following needs to happen:
1. Get into some therapy. A lot of the issues he saw were caused by my personal issues, which are improvable. (Mostly anxiety and worry and not all that much practice in relationships) Which leads to...
2. See someone else. Date, one-night stand, whatever. I can omit this if at least nine months pass and we happen to run into each other for NaNo stuff in 2013.
So that's that.