The big twist that comes about in line 13 of the poem is the new theme that makes its appearance: love. At this point in the poem, there isn't much new in the way of imagery; instead, we get the direct communication of thought and emotion between the speaker and his imagined listener.
- Line 13: Some people might disagree about this, but we at Shmoop think that line 13 of the poem is supposed to be a bit of a paradox. How so? Think about what the word "This" at the beginning of the line refers to: basically everything that has come before. If the speaker is saying that "thou perceiv'st" how he is sliding into old age and decrepitude, you might expect the listener to become disgusted by him and flee. So it comes as a bit of a paradoxical surprise to learn that seeing this "makes thy love more strong." How can this be? We'll have to wait for line 14 to find out.
- Line 14: The concluding line of the poem resolves (a.k.a. explains) the paradox introduced in the previous line: the listener will love the speaker even more now that he is on the brink of old age, because he realizes that the speaker won't be around forever. To emphasize this strong, positive emotion at the end of the poem, Shakespeare uses some incredibly complicated alliteration. First of all, you've got alliteration on L in the words "love," "leave," and "long." Then, you've got a second alliteration on TH in the words "that" and "thou." Next, you've got a third alliteration on W in the words "well" and "which." Also, it's worth pointing out that "love" and "leave" both also end with a "v" sound (in pronunciation), so that binds those two words closely together through consonance. This interwoven texture of sounds in the last line is probably a big part of what makes Sonnet 73 so powerful to this day.