Why am I dreaming about my ex if I'm over her?

For the past 3-4 days I've been dreaming of my ex gf for some odd reason even though I'm ove her now, she left me for another guy about a little over two years ago and she's still with the same guy apparently from what I heard (without me asking at all) so I don't know why I keep dreaming about her coming back to my life or last night which it was her boyfriend and her. Any help?