What is the psychological background behind this situation?

I have always wondered why when a man talks to his girlfriend about a girl he kind of dated before her, and maybe he has mention this girl a couple of times to his real girlfriend, then this creates a huge mystery surrounding this past girl he dated and a big curiosity for his real girlfriend to know more about this past girl he went out with.

Why does the girlfriend feel the need to see who the girl is, if she is pretty, hotter or more beautiful than her, it creates like an obsession, I have come across this so many times, it happened to me in the past as well, but I have always wondered, why does it create so much interest and curiosity to know who this past girl was and what she looks like and to even compare yourself to her, like if you needed to know everything about her, just cause she went out with the boyfriend in the past.

Is there some logical/psychological explanation to this or is it just based on insecurity due to the boyfriend mentioning this past girl and somehow this can create some sort of effect in the real girlfriend to find out and even compare herself to this past girl, even if it was nothing serious between them.