Bhagavad Gita, Chapter 2, Verse 11

The Deity said: You have been mourning for them who should not be mourned for. Yet you speak words of wisdom. The truly wise grieve neither for the living nor the dead.

You grieve for those you shouldn't. The wise grieve for no one, living or dead.

Arjuna sounds wise, but Krishna says sounding wise isn't the same as being wise. Someone who knows what lasts and what doesn't wouldn't grieve the way Arjuna is grieving now.