Because there's something which used to be between them and now it has died.
Lack of fulfillment and/or unsatisfied.
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
Your not getting what you need, and you feel powerless or are afraid to change things.
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.