We don't mourn the death of a dear one exactly.
I know it sounds weird but let me elaborate. We don't mourn the death of a dear one exactly, what we do mourn is the loss of a connection, a human contact/interaction, the window to our past, the missing pieces to the puzzle of our lives. That's it. That's what we mourn and feel sad about when we hear or experience the loss/death of a near and dear one.
Our soul misses the old times as it anticipates the future. Our subconscious mind drags us to all the long forgotten memories that we know exist but never have gotten time to cherish.
The places we couldn't got to visit, the memories we never were actually a part of physically and the most abstract concepts of our lives. All these things are lost when dear one is gone. And we mourn the whole package that person was. Not just a human it's the entire atom gone with all the sub-atomic particles. All the bonds lost and all the answers gone. The whole trajectory of your aura goes missing when you loose a certain intersection of your life's highway.