In Sonnet 15's first eight lines, the poet surveys how objects mutate — decay — over time: ". . . every thing that grows / Holds in perfection but a little moment." In other words, life is transitory and ever-changing. Even the youth's beauty will fade over time, but because the poet knows that this metamorphosis is inevitable, he gains an even stronger appreciation of the young man's beautiful appearance in the present time — at least in the present time within the sonnet. Ironically, then, the youth's beauty is both transitory and permanent — transitory because all things in nature mutate and decay over time, and permanent because the inevitable aging process, which the poet is wholly aware of as inevitable, intensifies the young man's present beauty: Generally, the more momentary an object lasts, the more vibrant and intense is its short life span.
Sonnet 15 also introduces another major theme that will be more greatly developed in later sonnets: the power of the poet's verse to memorialize forever the young man's beauty. "I ingraft you new," the poet says at the end of the sonnet, by which the poet means that, however steady is the charge of decay, his verses about the young man will keep the youth's beauty always fresh, always new; the sonnets immortalize this beauty. Ironically, the poet's sonnets serve the same purpose as a son whom the poet wants the young man to father: They perpetuate the youth's beauty just as a son would. In fact, the sonnets are even more immortal than a son. The sonnets continue to be read even today, whereas the young man's progeny may have completely died out.