I'd agree with Minin - if your partner is simply talking rather than ranting or looking whistful, he is simply exorcising his feelings.
The fact that he feels comofrtble doing this to you is actually a very big plus for your relationship - it shows he feels comfortable discussing his issues, and most men don;t do that easily at all.
Try and roll with it - if he gets obsessive, talk to him about why he thinks about her, but don;t discourage him from talking - most women moan that their partners don't talk enough!
If it's knocking your self esteem, try a few subtle hints about feeling a litle unloved, i am sure he will respond with some reassurance.