Why my ex dating my ex best friend who i hate?

Why my ex dating my ex best friend I hate a lot? She is deceives a whore and all her boyfriends and has tried to achieve with all my boyfriends that's why I stopped being friends with her and he knows how much I hate, but now he going out with her? Why?

Maybe each of them do not enjoy their company because they seem to be rather pessimistic and accusatory general.

My Best Friend’s Ex (Original Song, Acoustic)