Because they're probably in show off mode! As soon as their dominance is challenged, they get butthurt!
I listened to my ex talk about her failed relationships. I was okay with it.
bitching about others... it makes some feel taller
Mostly because men talk about how much he misses her, and what she did for him. While women talk about other stuff. Like how much "bigger" her ex was.... It bruises the ego a little you know.