He definitely had his fair share of insecurites. In both his long term mono relationships, the girl cheated on him, so it was somewhat understandable. The thing that always bothered me was that I was completely honest and open from the beginning so he knew what he was getting into. I know it was still hard for him because he had never experienced that type of relationship/situation before. I actually never really "ended" it. He talked with me over IM and basically ended it that way. He said he wanted to be friends and that it was too hard for him because it wasn't what he really wanted; which was to be with me and only me. It was all about the future with him. He wanted marriage and kids and to have a future with that person. I wish he would have figured that out earlier so everyone involved would not have gotten hurt. I guess the experience itself was something to be grateful for, even if it's ended somewhat badly.
As far as the new relationship chemicals, it's nice to hear that that's common. I felt like something was wrong with me because I had those feelings and couldn't just be satisfied with what I had. It certainly spices up the sex life though!