Why do i Occasionally have dreams about a former Best Friend?

All that you need to know; we were friends for 14 years prior - through primary school, High school and collage. She back stabbed me, for a guy online (it ended up the way i warned her about - Him vanishing), I tried to make things work between us, we became friends again but only for a short time; she did the same thing and just left me again for a new friend (also online!!) I have tried to mend and such; but nothing. I've accepted it and i no longer care about her anymore, i'm done with her pathetic. So why am i still having dreams about hanging out with her? (The old her, may i add. Ever since she met that guy; she had changed into a real different human being, someone who i no longer like) It's really irritating when i have these dreams.... i never care or think of her anymore, so why am i dreaming it?