Why we feel sad when someone dies?

I had been pondering about this why we feel sad when someone closest like parents die or others closer die even knowing we all will die at end.

Post my father’s death , I have been thinking about this and my scientific career, aptitude and philosophical nature I want to delve more into it as I know we don’t know much about death and grief than we know about life.

I want to ask this question and spur a discussion.

Looking forward for a matured discussion.