Sonnet 13 furthers Sonnet 12's theme of death by again stating that death will forever vanquish the young man's beauty if he dies without leaving a child. Some significance may lie in the fact that the poet refers to the youth as "you" in Sonnet 13 for the first time. "Thou" expresses respectful homage in Elizabethan parlance, but "you" expresses intimate affection. In any case, Sonnet 13 begins with the heartfelt wish, "O, that you were yourself," and the warning, ". . . but, love, you are / No longer yours than you yourself here live." This second line reminds the youth that at death, he will cease to possess himself because he has no offspring to perpetuate his name and his beauty.
The poet's proposal to his friend in Sonnet 13 contains ambiguities. Indeed, the young man may choose either to have a son or to remain only an image of himself when he looks in a mirror. Substance (a son) or form (the youth's image in a mirror) is the only choice presented. The young man seems so completely immersed in his own personality that his entire being is in doubt. Already the poet hints of deceit, which now the youth unwittingly uses against himself and later deliberately uses against the poet. By refusing to marry, the youth cheats himself of happiness and denies his continuation in a child.
The concluding couplet presents a new argument on the poet's part in persuading the young man to marry and procreate. Earlier in the sonnets (Sonnets 3 and 8), the poet invoked the young man's mother as a persuasive tool. Here, the poet asks why the youth would deny a son the pleasure of having the young man as his father, just as the young man found happiness in being the son of his father. And perhaps even more important, the poet questions why the young man would deny himself the rapture of fatherhood when he has plainly observed the joy of his own father's being a parent to him.