It's interesting how you start to idealize your ex after awhile.  Your unconscious carefully picks out memories, images, sensations, smells, tastes, emotions, and then uses all of them to sort of recreate a new person from scratch in your mind.  This new person so incredibly desirable, because all of the best traits that you remember about your ex are in them, fantastically exaggerated.  You long to see this person, to touch them, smell them, kiss them.  They feel like the one thing that you need, that's missing, that will fill an empty part of you.  But they're not.  Are they?  I don't know.  I think is important to be able to acknowledge when you don't know.  And I don't know.