What does it mean when you dream of dating someone you haven't talked to for a while now?

It clearly shows the earnestness of approach.

It means you are lonely.

Don't ignore the obvious when it comes to dream interpretation. If you are hungry before going to bed, and you dream of food, you don't need a shrink to delve into your childhood in order to explain it to you.

As to why you dreamt of him instead of someone else, the most likely reason is that lonliness is about isolation. Social and romantic longings share similar traits. So, if you are lonely, in either a social or romantic way (or both,) your mind is going to dig into the archives and find someone who fits the bill in order to tell yourself "Hey, forgetting something important? Just because you think you are busy doesn't mean you can ignore this."

Also, the mind tends to be semi-lazy when it comes to dreaming of what we want. If our subconscious desire is for a cabin in the woods, but we have never seen the cabin or the woods, we might dream of a house sitting alone in the suburbs because that is what we are aware of. Doesn't mean that is necessarily what we want, just closer to what we want than what we have now. So, this guy might not be the one but he's closer to the one (or your idea of the one) than anyone else.

You mostly dream about what is in your sub-conscious. It's understandable that you dreamed about him, because you were close with him in the past. It probably means you miss him (maybe in a friendly way) or just want to talk to him again.

Could be a desire that's been pushed out of the way. Otherwise, it could be working out some aspect of yourself that was dominant in your mind during that time & not have much to do with him. Just think of dreams as hashing things over & letting your brain fit everything together. It's a process that goes on & on.