Probably because you're thinking about them. Dreams are a result of your memories and thoughts you've learned/thought throughout the day. So, in large, dreams don't actually mean anything.
Now there are a few exceptions to this (such as dreams that express stress, anger, etc.) but don't mean anything specific like "you need to talk to them" or "you should go out with them" or anything of the such.
You dream about him/her because you had an emotional connection at one point with this person, and also I think you probably have some regrets about the relationship and how it ended. Maybe you should talk to your ex about your relationship if there is something bothering you, then maybe these dreams will stop.