Having explored the nature of his and the young man's relationship in the previous sonnet, the poet now returns to his theme of immortality. Not only does he grant the youth immortality through his verse, but because the poet's enduring love is repeatedly stressed as well, the poet himself gains a kind of immortality. Disclaiming kinship with the inconstant poetry of "painted beauty," he announces his only standard in the plea: "O let me, true in love, but truly write."
In Sonnet 21, the poet notes for the first time the presence of a rival poet; whether this is the same rival of later sonnets is unclear. Whereas in Sonnet 20 the youth's portrait was drawn from nature, in Sonnet 21 his appearance is concealed by cosmetics. Regretfully, the youth prefers inflated rhetoric and flattery to the poet's restraint, plainness, and sincerity. The criticism of the rival poet — "that Muse" — stems from the poet's view that too much hyperbole and artificiality indicate insincerity and false sentiment. Lack of sincerity, by extension, also is considered here an aspect of bad art. The poet criticizes the rival in a double sense, using the method of pretended understatement as a rhetorical device that contrasts the rival's superficial poetic style. Thus the phrase "fair / As any mother's child" sufficiently praises the youth, or anyone for that matter. But to say in the concluding couplet, "Let them say more than like of hearsay well; / I will not praise that purpose not to sell," reveals that the poet is himself engaging in a kind of excessive, elaborate, and affected eulogy. At any rate, the point of Sonnet 21 is that the poet speaks truth and the rival poet hyperbolizes.