What's he saying?
"But wherefore do not you a mightier way / Make war upon this bloody tyrant, Time?"
Why don't you work harder against the ravages of time?
"And fortify your self in your decay / With means more blessed than my barren rhyme?"
And ensure that your beauty lives on beyond the way I represent it in this poetry?]
"Now stand you on the top of happy hours, / And many maiden gardens, yet unset,"
Right now you are in your prime, and many virgin wombs,
"With virtuous wish would bear you living flowers, / Much liker than your painted counterfeit:"
Would gladly bear your children, who would look much more like you than a portrait of you:
"So should the lines of life that life repair, / Which this, Time's pencil, or my pupil pen,"
That child would fix your old age in a way that I, the poet,
"Neither in inward worth nor outward fair, / Can make you live your self in eyes of men."
Cannot, in any way, for written lines are not as good as an actual life.
"To give away yourself, keeps yourself still, / And you must live, drawn by your own sweet skill."
In giving yourself to a woman, you will create a new, young version of yourself in the children she bears.
Why is he saying it?
Sonnet 16 is a continuation of Sonnet 15, also of the "procreation" set. Though Sonnet 15 suggests that immortality can be reached through the poet's "engrafting," Sonnet 16 returns again to the theme of procreation. The final couplet of Sonnet 15 describes how the whole world is "in war with Time for love of you," and Sonnet 16 opens with a plea that the fair lord also defend himself against Time. The speaker calls his rhyme "barren," drawing attention to the fact that although it is one way to immortalize the youth, it does not do as much good as procreation.
In the first lines of Sonnet 16, the imagery of warfare enhances the idea of a battle against Time. In lines 1-2: "But wherefore do not you a mightier way / Make war upon this bloody tyrant, Time?" Time is called a "bloody tyrant" upon which the fair lord is encouraged to "make war" in a "mightier way" than merely being immortalized in verse, as was suggested at the end of Sonnet 15. The speaker urges him to "fortify" himself by having children to replace his youth.
A horticulture metaphor runs through lines 6-7: "And many maiden gardens, yet unset, / With virtuous wish would bear you living flowers." The "maiden gardens" refer to the wombs of virgins that could bear the fair lord's children. The phrase "yet unset" confirms that the women have not yet borne children. The "living flowers," therefore, are the children that would bear the young man's likeness. The personification of the gardens in describing them as having "virtuous wish" further enforces the metaphor.
The meaning of lines 9-10 is somewhat problematic, and there are various interpretations. The "lines of life" could refer to descendants in a linear heritage. But in light of the term "Time's pencil" in line 10, the "lines of life" could also refer to the wrinkles on an aged person's face, drawn there by Time. In line 10, "Which this, Time's pencil, or my pupil pen," it is unclear what "this" refers to. It could refer to the sonnet itself, but more likely, it is meant to be plural, or "these," referring to the two options other than having children: Time depicting you as you are now, aged, or the poet's description of you in verse. Both are inadequate, thus having children is preferable.
As the speaker encourages the fair lord to create new versions of himself in procreation, he uses the metaphor of a painter. The children will resemble him much more than a "painted counterfeit," or a portrait of him. Line 14, "you must live, drawn by your own sweet skill," suggests that the fair lord himself is the painter, or creator, of his children, or little replicas of himself. Thus will he become immortal, through his own doing, rather than that of the poet.