That feeling of emptiness, of something missing, of there being a dark hole, a void where something once was.
Mourning involves loss, traditionally the loss of another person in your life and the more important that person was, the closer the relationship with that person the deeper and larger the hole, the darker the void when we loose her, him.
People instinctively understand the how damaging, how traumatic the loss of a parent can be to a child. How that trauma can follow him or her. People understand that even as an adult that loss is never far away.
People instinctively understand that grief is a very normal reaction in these circumstances. They understand the damage loss can cause the pain that fills the void left behind.