Bhagavad Gita, Chapter 2, Verse 30

This, the Indweller in the bodies of all, is ever indestructible, O descendant of Bharata. So you ought not to mourn for any creature.

What lives inside every body can't die. That's reason enough to stop mourning.

This line ties together everything Krishna has said so far about the soul. Once its permanence is actually accepted, mourning for anyone starts to lose its footing.