Through Death and Life

Celestia ponders the nature of death, dying, and mortality after Rainbow Dash demands that she save Derpy from a natural death.

I am eternal, and death is my steadfast companion. The passing of each generation of my little ponies weighs on me; every one of them is a fleeting, irreplaceable treasure, a snowflake dancing 'round a fire. I want to save all of them; I have the ability to save a few. I have saved none.