I think it is inevitable that when we break up with someone, we compare ourselves to whomever they date next. Most of the time we cannot figure out why they would make such a terrible choice, or why would they go backwards so much. Sometimes they go for a big upgrade, but we are the only ones who cannot see it clearly. Sometimes we only see the faults in the upgrade, ‘oh he is really good shape. She likes to spend a lot of time with her man, and he is going to always be at the gym. That will never work’.
The phenomenon makes sense; we are too close to the situation to think rationally about it. The situation is complicated by a sex life as well. Jealousy over those who we have had sex with can be strong, especially in the aftermath of ego-shattering events like a break up.