As an early sonnet, Sonnet 17 fits with the "Fair Youth" sonnets that implore the young man to procreate. A recurring theme in these involves the poem's ability to preserve the beauty and memory of the beloved, even though time will exert its damaging influence on the person's physical body.
The conceit compares the poem itself to a tomb, enclosing the beloved's beauty and keeping it from others' appreciation. The quatrains continue to explore the theme of enumerating the many aspects suitable for admiration. Lines 6 and 8 both use polyptoton (repetition of a word but with a different meaning each time: "fresh numbers number" and "heavenly touches ne'er touched earthly faces"), creating a tension between the object of human beauty and the poetic beauty.
The third quatrain extends that thought by suggesting that the poem (or at least the paper on which it is written) is, like the beloved's body, subject to "age," a subtle form of personification, as the poem itself is projected to become like an old man whose words cannot be trusted.
The couplet resolves this dilemma of the poem's inability to command trust in future years. Should the beloved have a child, both his own beauty and the poem's record of it would be believed by means of the proof offered in the descendant's own beauty.
This particular sonnet is less overwrought than some, both in its rhetorical devices and in its insistence on procreation. As a result, the tone is gentler—not as plaintive or as despairing, nor as insistent about the poem's role in preserving the youth's beauty. Nonetheless, the dual focus on creation—both procreation and creative writing—mingles the hopefulness of the future with a melancholy tone over the inevitable decay of all living beings.