I know this is a common question on here but my mind is boggled.?

Why does my ex want to add me on facebook? heres a bit about the situation. first off she was my ex then we got back together after 8 months. But in hindsight, we werent really together because my love felt unrequited. We have a past we know each other very well. But one day she kind of disappeared so to speak, she wouldn't answer my calls or texts. so one day i called an d a guy answers and sends me a pic of him and her. she denied it, but i talked to her best Friend and she confirmed it. the last time we spoke we were at each others throats. i got a Friend request from her and told her that she hurt me very badly and i can not trust her anymore. whats her deal? why cant she leave me be?