For all their flaws compared to the dwarves and especially the elves, the humans in J.R.R. Tolkien’s world, as described in The Silmarillion, receive two courtesies from Iluvatar, the world’s maker, that even the gods themselves do not: They can act out of free will, and they will die.
It is one with this gift of freedom that the children of Men dwell only a short space in the world alive, and are not bound to it, and depart soon whither the Elves know not. […] But the sons of Men die indeed, and leave the world; wherefore they are called the Guests, or the Strangers. Death is their fate, the gift of Iluvatar, which as Time wears even the Powers shall envy.
The “Powers,” the Valar, are the 14 deities in charge of creating and steering Middle Earth and all its inhabitants—but even they only follow the will of Iluvatar. And though you’d think to gods, even the millennia pass like minutes, apparently, they do not. For the more entries and departures of other beings they observe, the more their creations are brought to nought by Morgoth, the evil one, and the more beauty and suffering they witness shine and fade, shine and fade, the more even the Valar long for a break.
How pesky, those humans! Appear for a few decades each at most, and yet they are the only ones who can pour either oil or sand into the gearbox of time all the same. And the height of frivolity? They won’t even be around to see the consequences, let alone suffer them! Meanwhile, the elves and Valar bear it all—an endless TV show without pause. Death a fate even the Powers envy? Ha! But time wears…and it wears.
Why do humans not want to die? Perhaps the main reason is that none of us have ever met someone who didn’t. If we talked to such a person, who knows? Maybe we might see death for the gift from Iluvatar it actually is. If even gods don’t want to live forever, the fact that our lives end may be one of the best things about them.