the reasons i can think of.
I feel that by dating my friend she is still somehow part of my life as we now have a mutual friend. Usually, When I move on from a relationship, I don't want to know what is happening in my ex's life, neither do I want them to know what is happening in mine. But now, through my friend she can still know everything about my life as she has her's all in my face. There are things he knows about me that I never told her because our relationship did not grow to that level. Now she might just get them all from him. It makes me feel naked.
I also feel like she is using him to say to me "You should not have thrown away a rare gem like me". She can be a bit of a vindictive attention seeker like that.
Another thing is People keep asking me why my friend snatched my girlfriend even though I was the one who pulled the plug on the relationship. Everyone does not understand why I would not be jealous. But I am not because I fell out of love with her before the relationship ended. Still, now I wish they were not dating because answering these questions is just exhausting.
Things like this happen in life, I guess. We only have control over our own individual lives, not other people's.