These types of dreams are fairly common, and more so among younger people. There is not much to it other than your brain dealing with emotional flotsome. "Ex's" are usually the other person in the dream because it is easier for your brain to fill character slots with people you already know. There is already some emotional attachment that does not need to be explained in any other way.

As for the reason for the dream....There are any number of emotional triggers that can cause this. I would ask if you have been thinking of the past alot recently or thinking alot about relationships and emotions like love? If you think about what you have been dwelling on lately you may find the root for the dream.

But as far as it meaning anything I don't think so because like I said it is a fairly common dream type.