Couplet Summary

This thou perceiv'st, which makes thy love more strong,

• With line 13, we move into a different phase of the poem: the final couplet. Where the first 12 lines of the poem (divided into quatrains 1-3), impressed us with their Rubik's cube-like ability to take a topic (Life/Death/Aging) and twist it around into new shapes and colors, the couplet's job is to take all of that stuff and (a) sum it up, and (b) add a new twist. 
• So that's the challenge. Let's see how line 13 meets it. As far as summing up goes, it looks to us at Shmoop like Shakespeare does a pretty handy job of this in the first three words of the line: "This thou perceiv'st." This works as a way of summing up in two ways. First, there is the simple repetition of one of the main ideas of the poem: sight. So the verb "perceiv'st" in line 13 echoes not only the preceding "seest" from line 9, but also the "seest" in line 5, and even takes us all the way back to the word "behold" in the first line of the poem.
• We see the second way the line acts as a summing-up device when we ask, "What is it that 'thou perceiv'st'"? In other words, what does the "This" at the beginning of line 13 refer to? 
• If you ask Shmoop (though you may have a different interpretation), the word "This" at the beginning of the line really refers to everything that the speaker has talked about before; in a sense, "This" refers to the preceding twelve lines of the poem. 
• You could almost imagine that the speaker at this point is, literally, imagining that the "thou" he is talking to is reading this very poem, the one that you Shmoopers are currently reading. Pretty cool, don't you think?
• Okay, so that's how line 13 gets the idea of summing-up in there. As for the twist, that comes in the second half of the line. Any guesses what it is? 
• Yep, "love." For the first time, Shakespeare makes clear the nature of the relationship between the person speaking and the person he is speaking to. 
• Not only that, but he makes a connection between the previous twelve lines and the new theme he's introducing. Perceiving all these things, the speaker says, will make the love of the person he is talking to "more strong."
• Why will this happen? Let's move on to the last line of the poem to find out.

Line 14

To love that well which thou must leave ere long. 

• Now, we get the final payoff of the poem. The speaker is telling the listener that not only will their love "become more strong" when they realize that the speaker won't be around forever, but they'll also love him "well," i.e., they'll cherish him all the more. 
• In this way, Shakespeare neatly ties the themes of love and death together into a single complex idea. Way to go, Big Willy, way to go. 
• This sense of the special preciousness of those things that don't last forever is a common theme in Shakespeare's sonnets, though it is usually counterbalanced by calling attention to things that are slightly more permanent, like, say, poems. 
• Poems, of course, only last as long as people read them. So, if you liked Sonnet 73, why don't you help his other poems exist by reading some of them. And as you do, you might try to keep in mind the themes and technique of Sonnet 73, so you can see how other poems play and experiment with this same material, which they totally do, and just as well.