Ex boyfriend/girlfriend back together- can such relationship still grow at all?

Well I guess it depends on who the ex is....... My ex dumped me after 3 months then two days later we were dating again after he said he still liked me and made a mistake letting me go but after a two months he dumped me again and has disliked me since for no apperent reason. So I don't think exes should get back together because if they broke up once wouldn't they just break up again?