Why wouldn’t he? I think it’s a nice gesture.
Just because a relationship ends doesn’t mean a person stops caring about his ex. When you spend that much time with someone and get that close to them, you can’t help but have a fondness for the person (unless the breakup was muy ugly). Exes share something no one else shares: a specific period of time when their lives intersected and they were the most important people to each other. Nothing can ever change or take away that small piece of mutual history — it belongs solely to them.
I’ve been married a long time, but I still have warm fuzzies about some of the women I dated before I met my wife. I think about what attracted me to them, what I enjoyed about them, the good times we had, why I loved them, and what I learned about myself from them. As your boyfriend points out, each relationship in our past has helped mold us into who we are today, and he feels grateful to this particular ex for improving him. How many of our exes have ever thanked us for such a thing?
Does that mean he wants to date her again? No. If that’s your concern, don’t confuse fondness for a shared past with a desire to reconnect. After all, he said you are perfect for him, right?
I also get a sense of guilt from your guy about his relationship with this ex — he’s a “changed man” and a “better boyfriend” than he was to her, which suggests that he knows (or thinks) he made a lot of mistakes with her. His healthy relationship with you has opened his eyes to his failures with her, and the e-mail is his way of apologizing to his ex without coming right out and saying he’s sorry, which is hard for anyone.
A third possibility is that he’s taunting her: you’re perfect for him (she wasn’t). He recognizes good in you (because she was so horrible). He’s a changed man (happy with you, miserable with her). But I don’t think that’s the case here. That would be some hard-core mind-f**king on his part, and few people are that sadistic. If he is, run.
No, my gut tells me that this guy loves you and loves the way you make him feel about himself, but he has some regrets about his ex and felt compelled to thank her and pseudo-apologize to her because he’s a good guy.
I could be wrong, but why not assume the best in people until they show you otherwise?