That time of year thou mayst in me behold When yellow leaves, or none, or few, do hang Upon those boughs which shake against the cold, Bare ruined choirs, where late the sweet birds sang. In me thou see'st the twilight of such day As after sunset fadeth in the west; Which by and by black night doth take away, Death's second self, that seals up all in rest. In me thou see'st the glowing of such fire, That on the ashes of his youth doth lie, As the deathbed whereon it must expire, Consumed with that which it was nourished by. This thou perceiv'st, which makes thy love more strong, To love that well which thou must leave ere long.
I always had a too clever by two (2x2 clever) reading of that last line: that the word "leave" also meant putting out leaves or putting leaves on to, and not just "abandon" or "depart from." I didn't like this facile vulgarization, but it was the only way I could think of to approach the paradox that it's the young man who's forced to leave at the end of the sonnet, not the dying Shakespeare.
Facile vulgarization, yes, because the words to have in mind are Antony's sublime "Let that be left which leaves itself." Being left, leaving: the abandonment there is what matters and is only blighted by imposing a stupid pun (as opposed to a kind of homonymic echo) on to the word.
But what would the young man be leaving? I think we have here a precursor to Wordsworth's Resolution and Independence. It is the nature of life, or of an aesthetic life, the life of those who would agree with Deleuze when he says, "An indescribable joy always rushes out of great books, even when they speak of ugly, hopeless, or terrifying things,” that poets in their youth begin in the gladness of being able to feel this joy, the gladness of anticipating horror and writing about it with all the gusto or brio that's the obverse of even the most melancholy intensity. "Thereof in the end come despondency and madness," Wordsworth comes to realize, as despondency comes to seem real to him. Mary Shelley will note something similar in her preface to the third edition of Frankenstein, after Percy's death, and the death of so many of her little ones:
And now, once again, I bid my hideous progeny go forth and prosper. I have an affection for it, for it was the offspring of happy days, when death and grief were but words, which found no true echo in my heart. Its several pages speak of many a walk, many a drive, and many a conversation, when I was not alone; and my companion was one who, in this world, I shall never see more. But this is for myself; my readers have nothing to do with these associations.
Her hideous progeny is the book, not the monster. She describes death and grief but is immune to it when writing the book: these are words which find no echoes and so they yield a Deleuzean joy and Wordsworthian gladness. Does it prepare you for the future, this flower-guided dallying with death (cf. Frost, Coleridge)? That's the question in these poems. Wordsworth dallies with the very fact that the gladness he feels in knowing that life is a life of despondency and madness is going to be self-defeating, self-undermining. That's a glad realization too, to the poetic, rejoicing figure of youth he was at the time.
What about the young man then? I think that the last line of the sonnet describes the merger of aesthetic and real experience. By aesthetic experience I mean what Wordsworth means: the inspiration to poetry through the contemplation of the miseries of life since those miseries measure the possibilities and depths of human dignity and human experience. So Shakespeare shows the young man the curtailments of time (life, year, day, fire at night) and what the young man can "behold and see" (the sonnet echoes Antony and Cleopatra) can yield authentic perception.
Such perception makes the young man's love of life which, like everyone, he must leave ere long ("All lovers young, all lovers must / Consign to thee, and come to dust") the stronger. Stronger not because it is more precious, or not only because it is more precious, for its scarcity. But stronger because the grimness of life gives it depth, and the depth of life is what we love about it.
Or love at any rate about literature, about the depth that literature can achieve. That achievement, in Sonnet 73, redounds to the merit of its speaker (or poet, to the merit of Shakespeare). In telling the young man that he too must die, and telling it in such a way that the young man loves the world which produces poems like Sonnet 73, Shakespeare shows us two ways of thinking deeply: writing well, as the poet does; reading well, as the young man does. Naturally this is the structure of drama as well, and there's a sense in which all the sonnets are reflections on Shakespeare's thought about play-writing: "As a decrepit father takes delight / To see his son perform the deeds of youth," e.g. (though there the positions are reversed).
Resolution and Independence, then, would be the young man's answer. (I am certain it is. I am certain that an enormous number of Wordsworth's poems are haunted by Sonnet 73. Consider: "Seals up all in rest" and "A slumber did my spirit seal.") Here too the young man loves what he must leave ere long, but it's the young man who's the poet, and the leech-gatherer who's just a leech-gatherer. The burden that Wordsworth takes up, or has taken up, is that the youth is both poet and audience of his own coming dilapidation. It's not that dilapidation will put him in a position to speak of such things to an audience that will thereby fall in love with the depths of life. It's that falling in love with the depths of life is something we poets do in our youth. We fall in love with the fact that futurity holds despondency and madness, and we speak of such things as part of our youthful vocation (no danger that Shakespeare's young man has such a vocation), a vocation which leads to just these things.
Writing, to use the word Beckett and Blanchot hit upon independently, is a serious task, and only youth woud be foolhardy enough to undertake it. Only youth or Shakespeare, thinking dramatically, thinking about what he owes to a younger generation that will need a measure of the depth of human sadness, in order to be able to offer such a measure themselves when they too come to be old.