“Thy” in this final line refers to the youth. This is the first time in this sonnet that the speaker has directly spoken to or referenced the addressee, the fair youth. The rest of his arguments about death, Time, and the human life cycle are abstract concepts. This offers one reason why the speaker can be more objective and rational about Time and aging in this poem: he is not imagining them as directly threatening his beloved.
With this reference to the Christ narrative, the speaker likens “maturity” or adulthood to suffering. However, he also creates an implicit connection with the idea that suffering brings redemption. Unlike the previous sonnets in the sequence that considered aging and maturity a violent tragedy, this sonnet seems to consider aging to maturity a necessary process.
Scholar Stephen Booth points out that the logical structure of the first quatrain imitates its own content—the movement of the waves. According to Booth, the first two lines form a combined unit which is then modified by the third line. As the reader encounters the notion of “each changing place,” her understanding of the waves/minutes in the first two lines is retroactively altered. Arriving at the fourth line, it becomes clear that the phrase “each changing place” actually serves as an adjectival modification of “all forwards.” Once again, the new line alters the reader’s understanding of “that which goes before.” Each successive line “chang[es] place with its predecessor.” As Booth puts it, the “physics of the quatrain” imitates the physics of the described wave pattern.
