After people have left, all we have of them are memories. The scars they left are usually the most profound, we remember them more clearly, and they overshadow the joyful memories we had. It's most likely our minds are trying to remind us of the endings that our emotions may have forgotten. Our minds are trying to keep us safe and secure. Those horrible recollections of how it ended, of how broken and wounded we felt, are our protection against ever wanting them back.