Women have a fear that their ex will change for the new woman.
Men don't work that way. In most cases, they become worse. When a break up happens, the only lesson that men learn is that they should find a woman who will put up with their most toxic traits. They don't want to have to deal with another nagging woman. This is why the more relationships they've had, the quality of the new person goes down.
Women, on the other hand, do the exact opposite. They will put up more with the new man. Things that they would have never let slide with their ex. This is why the more relationships they've had, the quality of the new person goes down.