Why did Tolkien call death a gift?

In The Silmarillion, he writes death is an exclusive gift given to mankind by God. All other creatures envy this gift, including the immortal elves:

Mankind alone, through death, is granted union with the divine.

Tolkien's point is that immortality in a fallen world is not a blessing, nor man's actual purpose. To live forever in a world marred by corruption, vice, and decay is to be trapped with no escape.

Death, then, is not a tragic ending, but a release — a return of creation to its creator. The humility of mortal man leads to a glory far greater than immortality.

In other words, man was made for something greater than earthly pleasure. Death is the preparation for eternity.

Today we tend to see this backwards. We treat death as the ultimate evil, and endless life as the ultimate good, no matter the cost. We try to preserve life indefinitely, and in doing so, lose sight of what life is actually for.

Tolkien's insight is this:

A man who refuses to die for anything will one day find he has nothing worth living for.

A world that fears death above all else will never reach the highest good, for life truly begins when you discover a love greater than life itself.