What's he saying?
"O thou, my lovely boy, who in thy power / Dost hold Time's fickle glass, his sickle, hour;"
Oh you, my lovely boy, who hold in your power Time's fickle hourglass (or mirror), his sickle, and his very hours;
"Who hast by waning grown, and therein show'st / Thy lovers withering as thy sweet self grow'st;"
You who have grown as your youth has declined; meanwhile, your lovers have withered as your sweet self grows;
"If Nature, sovereign mistress over wrack / As thou goest onwards, still will pluck thee back,"
If Nature, the controller of destruction, will continue to keep you back in the sweetness of youth even as you age,
"She keeps thee to this purpose, that her skill / May time disgrace and wretched minutes kill."
She is keeping you for a reason, so that her power may disgrace time and cancel the wretched effects of time.
"Yet fear her, O thou minion of her pleasure! / She may detain, but not still keep, her treasure:"
But fear her, oh you servant of her pleasure! She may delay the decline of aging, but she cannot keep your youthful beauty forever:
"Her audit, though delay'd, answer'd must be / And her quietus is to render thee."
Time's reckoning, though delayed, must still be settled, and her reconciliation will be to give you up.
Why is he saying it?
Unique in the sequence, sonnet 126 is actually not a sonnet at all, but rather a verse of six rhyming couplets adding up to twelve lines. Nevertheless it is still possible to analyze this "sonnet" quatrain by quatrain, since each four-line block constitutes its own thematic unit within the overall theme of the fair lord's preternatural resilience to the ravages of time. The attitude of the sonnet is not jealousy, as we might expect, but rather admonition: the fair lord's resistance to time's destructive force is ironically (or sadly) just a temporary blessing.
In the first quatrain, the narrator admires his "lovely boy" for the superhuman power he seems to possess over Time's various instruments of destruction. "Time's fickle glass" in line 2 may be an hourglass, but it could also be a mirror - for a mirror shows the present, unlike a picture that shows the past, and thereby a mirror shows the changes that have taken place with time. For the fair lord, however, these changes have yet to detract from his beauty, as lines 3-4 show: "Who hast by waning grown, and therein show'st / Thy lovers withering as thy sweet self grow'st."
The second quatrain identifies Nature as the fair lord's generous accomplice, for it is Nature that has granted him his resilience against time by continually rescuing him from time's destruction. This comes as little surprise, if we have read in sonnet 20 that Nature has been in love with the fair lord all along. She therefore saves him presumably for her own gratification, as we see in the opening of quatrain three: "O thou minion of her pleasure!"
The final quatrain delimits the fair lord's specious immortality, as line 10 warns that Nature "may detain, but not still keep, her treasure." His fate is forever sealed in lines 11-12, one last example of financial imagery in the fair lord sonnets, where Nature's "audit" of life and death must be reconciled by the eventual termination of the fair lord's earthly figure: "Her audit, though delay'd, answer'd must be / And her quietus is to render thee." (The words "quietus est" were written atop acknowledgments of settled debts.) The power of Nature may be great, but it is unable to withstand the ravages of time indefinitely.
One of the most heated debates surrounding the collection of Shakespeare's sonnets is the question of what deeper significance, if any, is to be found in their ordering and internal structure. How deliberate is the ordering of the sequence, and to what extent are we able to divide the sonnets into groupings and subgroupings? As mentioned elsewhere in this ClassicNote, the primary division most scholars make comes between the fair lord sonnets (1-126) and the dark lady sonnets (127-154). Sonnet 126 is often viewed as the definitive breaking point, for its aberrant "non-sonnet" structure seems to be evidence of the poet's insertion of these lines as an explicit "curtains close," or at least as some sort of meaningful interlude. Sonnet 126 is the narrator's final farewell to the fair lord and also his final admonition, reminiscent of the prophetic epigram of sonnet 60, that Time "Feeds on the rarities of nature's truth / And nothing stands but for his scythe to mow."