Shall I compare thee to a summer’s day? Thou art more lovely and more temperate (1-2)
At the start, we’re told that this will be a poem putting man and the natural world side by side for comparison, and apparently an attempt to favor man over the summer’s day. Let’s see how that develops:
Thou art more lovely and more temperate: Rough winds do shake the darling buds of May, And summer’s lease hath all too short a date: Sometime too hot the eye of heaven shines, And often is his gold complexion dimm’d (2-6)
At first glance, these lines look like the poet is bashing summer, in favor of "thee," who is "more lovely and more temperate." That’s true enough, as he’s definitely complaining about summer, but pay close attention to how he words these complaints. He says that winds "shake" "darling buds," that summer has a "lease," that heaven has an "eye," and that that eye, the sun, has a "complexion," which implies a face. What do all of those quoted words have in common? They’re all ways of describing humans or their actions. In other words, he can only describe nature by personifying it. This is especially pointed in the words "temperate" and "complexion," since they can both describe states of human health (at least as understood in Shakespeare’s time). If you had a good temperament and complexion, your internal humours, or various liquids, were in good proportions. So basically, even if nature is a little less lovely than a human, they clearly are made of the same things and subject to the same rules.
And every fair from fair sometime declines, By chance, or nature’s changing course untrimm’d (7-8)
Here the poet makes the similarities even more clear. He’s stopped trying to differentiate between man and nature at all, and instead points toward their universal similarity: they’re both slaves to time.
But thy eternal summer shall not fade, Nor lose possession of that fair thou ow’st, Nor shall death brag thou wander’st in his shade, When in eternal lines to time thou grow’st (9-12)
While the opening lines suggested that the poet would pit man and the natural world in competition, here we realize that the poet is placing man and the natural world together against time. It turns out, as we saw in all of the previous lines, that man and nature actually have tons in common, and even if the poet set up the comparison to find differences, he mainly finds similarities. Changing tack, then, he groups man and nature together in a fight against fate and time.
So long as men can breathe, or eyes can see, So long lives this, and this gives life to thee (13-14)
By the end of the poem, poetry itself has become part of the natural world, as the poem’s continued "life" is the key to keeping "thee" alive.