Youth is like an uncollapsed wave function.
Much that is alluring in a youthful thing—a child, a young woman, the bud of a rose, even a fresh morning—is that each has the potential to become so many forms of itself; that, in a way, all those possibilities exist at that moment in superposition.
We can imagine any number of futures for a young man: he can become all sorts of people, have all sorts of experiences. His life, in its youth, is like a sheet of paper not yet written on. His mind, his face, his skin, is fresh and uncreased by striving, unwounded by living.
Yet, the older something becomes, the more the possibilities for what it can still become are cut off. Until we, and it, narrow down to death.
Thus the sadness of life. For, when it is spent, the observer sees that, at that moment of youth, the path that will be taken can only ever be the one that was taken.