This passage is from an essay called "The Temporary Universe" by Alan Lightman which can be found in his wonderful book The Accidental Universe: The World You Thought You Knew.
"...In my continual cravings for eternal youth and constancy, I am being sentimental. Perhaps with the proper training of my unruly mind and emotions, I could refrain from wanting things that cannot be. Perhaps I could accept the fact that in a few short years, my atoms will be scattered in wind and soil, my mind and my thoughts gone, my pleasures and joys vanished, my "I-ness" dissolved in an infinite cavern of nothingness. But I cannot accept that fate even though I believe it to be true. I cannot force my mind to go to that dark place. "A man can do what he wants," said Schopenhauer, "but not want what he wants."
"Suppose I ask a different kind of question: If against our wishes and hopes, we are stuck with mortality, does mortality grand a beauty and grandeur all its own? Even though we struggle and howl against the brief flash of our lies, might we find something majestic in that brevity? Could there be a preciousness and value to existence stemming from the very fact of its temporary duration? And I think of the night-blooming cereus, a plant that looks like a leathery weed most of the year. But for one night each summer its flower opens to reveal silky white petals, which encircle yellow lacelike threads, and another whole flower like a tiny sea anemone within the outer flower. By morning the flower has shriveled. One night of the year, as delicate and fleeting as a life in the universe."