Possibly because your relationship was never completed...it never ended and it was never let go...
Sometimes we love something so much that we have to let it go...if it comes back it was always meant to be, if it doesn't, it never was...sort of an old saying...and at the best of times, we just cannot be with this person however in another lifetime that person shall be seeing you again...
I am wondering if it is possible that you two were connected in another lifetime?...past lives or relationships do in some way come back to us to either complete where we finished off in that lifetime...it may have been something that was not right back then, but in this lifetime it is not completed and still weighs on you...
Sounds like you are both still connected for you to continue having dreams of him...what a wonderful dream though...it is filled with love, a magical kind of love...
LL
Angie