This is because once you are fond of the person and you get fond of his/her company, there is some sort of attachment that binds you together. You cannot easily forget the person because of the bond that was formed when you were in love. Therefore, as much as the person is out of your life for good, that special bond still remains.
We were created to Love. Not out of sinful or lustful desire, our minds are often our downfall. We never forget stuff, only learn to focus on more important issues. We forget the hurt and pain. Learn to do that