Oh, I gave this an answer in the other section. I'll paste it here and delete the other one:

I guess that sometimes our brains are like old filing cabinets and our subcoscious goes through all the folders from time to time, then comes across a file with a person inside you haven't seen in ages and thinks it wouldn't mind an update so it makes you dream about them - and of course it would be difficult to contact them because you don't have their number or know where they live, and you probably aren't sure what they even look like now.

I suppose if it keeps happening you could always try to get in touch with them to resolve the dream?

Or maybe it's more that you feel something is missing in your life romantically, and your dreams are thinking back to a time when you didn't feel this way - or at least the way you remember it you didn't.