The three quatrain plus couplet form of Shakespeare’s sonnets also allow the speaker to use a dominant metaphor or image for each quatrain as he leads to his resolution in the concluding couplet. Each quatrain then contains a certain logic of its own. In Sonnet 18 the first quatrain argues the beloved is more gentle than the season of summer, which can be harsh and brief; the second quatrain becomes more specific, using the metaphor of the “eye of heaven,” which is the sun, to argue that summer can reduce beauty. This quatrain concludes with a colon, signifying a shift in thought in what follows, which we see with the “But” that introduces the third quatrain. Here the speaker extends his images from the season of summer (quatrain 1), to the sun (quatrain 2), to time and eternity—the beloved’s beauty will never end. Why is all of this so? The couplet provides the answer, which is that his beloved’s beauty and life exist in the words the speaker gives them; the beloved exists outside time altogether because of the poem which “gives life” to him or her.
As you'll find in the analysis for this sonnet (see link below), the way Shakespeare would pose a rhetorical question in his sonnets, then only allow for one answer (his own), was important in getting the point of his sonnet across to his reader, and most especially, to the intended recipient of the sonnet.
Shakespeare used the traditional sonnet form for all but three of his sonnets, which is 14 lines, broken into three quatrains (stanzas with four lines) and one concluding couplet (two lines with end rhymes). The rhetorical question is typically posed within the first quatrain, then expounded upon through the other two quatrains, and finally, a conclusion/answer is offered in the couplet.
This form - 14-line sonnet in iambic pentameter - was very conducive to the meaning of the sonnet. It works particularly well with Sonnet 18, as he is positing the fact that his beloved, when compared to the beauty of nature, is far more lovely, more calm, etc. Shakespeare is also making the point that his beloved can be immortal, despite the usual deteriorating effects of aging and nature. He presents this in the quatrains, leading up to his final point in the couplet, which reads:
"So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee."
He is saying at the end that as long as his sonnet exists, and as long as people are still living and can read it, his beloved will be immortal through the lines of his sonnet.