Originally Posted by Jinn

I think it's all been covered, so I'll add a +1 for "insecurity" as the answer the OP's question of "What makes us jealous of someone's ex."

If you think you're the hottest shit in town, you are unlikely to ever be jealous of someone's ex. That fact alone makes it pretty clear that jealousy is directly proportional to insecurity. The easiest way to combat it is to realize that you've already won, in the sense that you're with him now. And to realize that they're lucky to have you, rather than vice versa. What his ex-girlfriends looked like, did, thought or believed is really not your concern if approached from this angle.