The rhyming couplet comprises the last two lines of Sonnet 18, lines 13 and 14.
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
In the couplet, the sonneteer is saying that as long "as men can breathe or eyes can see," Sonnet 18 will live and be read, and, in so being, will immortalize the subject of the sonnet, which is the speaker's beloved. [Some assert that Shakespeare is writing of his own loves while others suggest Shakespeare is simply writing a sonnet cycle that is in many respects not autobiographical.]
The first two quatrains answer the introductory rhetorical question: "Shall I compare thee to a summer's day?" Line 3 and 4 of the first quatrain and lines 5 through 8 of the second quatrain explain the "Why" of the implied "No, because" that comprises the rhetorical question’s answer in line 2: "Thou art more lovely and more temperate" than a summer's day.
The essential "why," or explanation, offered in lines 3 through 8 is that while winds, sun, and "nature's changing course" may destroy summer's beauty (turning it in fact to autumn), nothing will destroy the beauty of his beloved. The "because" that follows the first two quatrains is in the third quatrain and is that the sonnet will immortalize the beloved’s beauty and life.