Because there's something which used to be between them and now it has died.
Lack of fulfillment and/or unsatisfied.<br />
I had the same feeling in my last relationship - WHILE i was in the relationship. Once you can fill that void in your heart, you'll know :)
maybe that someone is realizing that no outside thing can make him or her happy
Not being able to bond/communicate with the person the way your subconscious wishes. Only you know what would truly make yourself happy in a relation.