First, the text.
Shall I compare thee to a summer's day
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.
Sometiems too hot the eye of heaven shines,
And often is his gold complexion dimmed;
And every fair from fair sometimes declines,
By chance or nature's changing course untrimmed.
But thy eternal summer shall not fade
Nor lose possession of that fair thou ow'st;
Nor shall Death brag thou wanderest in his shade,
When in eternal lines to time thou grow'st
So long as men can breathe or eyes can see
So long lives this, and this gives life to thee
The first line establishes immediately what a professor of mine called "the poetic contract." Quite simply, Shakespeare tells us what his poem will be about. He is going to compare his amore to a summer's day. With lines two and three we realize that the rest of the poem will expand and extrapolate the answer. The second line establishes the manner in which the question will be answered. The sonnet is virtually synonymous with poems about love and lovers, so from that we also know that not only will the answer to the question in line one be "yes" (yes, I will compare you to a summer's day), but the answer will come out quite in favor of the lover over the aspects of the object the lover is being compared to.
That would, in fact, be the traditional idea as executed by a competent Hallmark card writer. However, as the poem goes on we realize that Shakespeare's sonnet plumbs and climbs multiple levels until finally the poem isn't just about love and lovers, but about decay and the poem itself.
"Temperate" and "lovely" are the key descriptors of lines one and two. Everything's calm, the skys are blue, but then we have a bit of a wind picking up in line three, a cool stiff breeze perhaps that "shakes the darling buds of May". Deployed here, the wind efficiently reminds the reader that fall - or is it The Fall - even in late spring is never too far off or too far behind. Or, as the next line notes so well "summer's lease hath all to short a date."
So what we have here at the 1/4 way mark is a temperate poem of love with hints of mortality setting in. One might expect the rest of the poem to progress steady on from summer to fall to winter to a "how I will cherish growing old with you" progression, which is a great progression that's been done well by many poets including S himself. But lines 5-9 shift the poem decisively away from this agenda.
Where we had hints of an approaching fall early on, lines 5-6 stop the poem squarely in the hot hot summertime. Summer, tho it may seem like a time of paradise in the dead of winter, can be so warm, so sweaty it seems that cooler air or evening will never arrive. And the clouds that dim the "gold complexion" do nothing to alleviate. Time is very very still all of a sudden in lines five and six. The light is very bright, very hot and that heat comes to bear on the next line "every fair from fair sometimes declines".
For a sonnet in particular, the tone of this line surprises with its tone of resignation. But care must be taken here to remember that line is part of a phrase which continues into the next. The full phrase reads "every fair from fair sometimes declines/ By chance or nature's changing course untrimmed." The question to ask about line 8 is "what is 'nature's changing course?" There is perhaps a bit of confusion. After all, nature doesn't change. There are the same cycles of seasons year after year. It is nature's nature to change and it is that nature that is "untrimmed", unaffected by sail or wind. This aspect of nature cannot be cut, shaved, shorn - not even by Nature itself. Time will march on, beauty will change in decay. It will not last. We can look at Apollo's lovely torso, but time and decay did remove those arms. Nature ain't gonna change, you're gonna get ugly, you gonna die...
And then line 9 comes along, the most famous "but", the most famous literary rescue of all time. We're still in that loonggg summer. Suddenly, though, it's not so hot, the cloud's aren't stuffy. That famous b-u-t offers a promise of alleviation, a promise of hope that the march of time can be staved off, that beauty and love can somehow remain unaltered. I can almost hear you pant "how's that? How can it be done? Where is this fountain of youth?"
Easy now. We're getting there. Patience. What is "that fair thou ow'st"? I'll answer first and then explain. What is owed - that is, what must be given back gradually is the life and beauty first granted us. Turn the word "fair" into "fare" and what do you get? With fare you get a debt you must pay. With "fair" you get the nature of the payment, your "fairness" i.e. your beauty. In larger terms, you owe the life you are given. And that debt you pay gradually over the years. You decay gradually, you die.
To make matters even worse, who is it we owe this "debt" to? We owe it to God. This is according to Shakespeare himself. Prince Hal to Falstaff in Henry IV: "We owe God a debt": to which Falstaff says ""Tis not due yet, Hal. 'Tis not due." That word "debt" looks, sounds suspiciously and scarily like "death". Probably have common origins as words, too. Things are just looking very, very bleak for us in terms of salvation from the ordinary rut of decay.
But...that famous "but" again...suddenly the poem itself comes into play. Shakespeare's betting salvation on the time-worthiness of his lines, his writing. 400 years before post-modernism. 400 years before modernism? Heck, this was 400 years before "posts". And in that spirit I the Author of this piece will not hide behind the coy affectation of Shakespeare employing a "narrator." This is Shakespeare making the wager. And he's betting on the greatness of his poem to stir the minds of readers, critics, biographers, historians across every century since to wonder, passively or actively, "who was this love?" Who is this love who withstood the rough winds, of whom Death could not brag "wandered in his shade, whose "fair from fair" could not decline. I prefer to settle it in my own mind with the fact that five centuries later not only do we wonder about this lover in a dreamy, starry-eyed way, but also with one of the great and very hot literary controversies still raging. Shakespeare bets. Shakespeare wins. Lucky bastard.
Luckily, we still have that famous "but".