Bhagavad Gita, Chapter 2, Verse 27

Of that which is born, death is certain, and of that which is dead, birth is certain. Over the unavoidable, therefore, you ought not to grieve.

Birth guarantees death. Death guarantees rebirth. Grieving over something guaranteed doesn't make sense.

People act shocked by death even though it's the one thing every living being is guaranteed to face. Mourning something this certain is like being surprised every single time it happens.