Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove:
O, no! it is an ever-fixed mark,
That looks on tempests and is never shaken;
It is the star to every wandering bark,
Whose worth's unknown, although his height be taken.
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come;
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.
If this be error and upon me proved,
I never writ, nor no man ever loved.
It's difficult for any intelligent person not to hate Sonnet 116 on account of all the damage it has done in the service of dull wedding ceremonies ... difficult, but not impossible. Underneath all the historical crust of accrued schmaltz, there is a respectable, functional sonnet.
The first line alone is a masterpiece of metrical license taken just to its limits:
let me | NOT to | the MAR- | riage of | TRUE MINDS
What have we got there? The way I've scanned it, which is by no means definitive, but seems reasonable to me, we've got a pyrrhic foot (two unstressed syllables) at the outset. If you like, it's a garden-variety initial inversion--a trochee (stressed, unstressed) for an iamb (unstressed, stressed)--but I have to strain to perceive any appreciable syllabic emphasis at all. Then, directly following that, we've got a trochee. So whether other readers agree with me about the pyrrhic opening or not, we've at least got solid trochaic meter for the first two feet. In the third foot, we get our first iamb--and our last, if you'll grant me another pyrrhic foot in the fourth slot. Finally, we close with a spondee (two stressed syllables). One unequivocal iamb in the whole line.
No big deal by itself, perhaps, but then in the second line we've got that strong caesura (effected by the period) after the first two words, another pyrrhic foot in the third foot, and a trochee in the fourth foot. For the first half of the first quatrain, the sonnet is basically indistinguishable from prose. The second half of the stanza (indeed, most of the rest of the poem) is in mechanically regular iambics, and we have to project this regularity backwards to hear how the rhythms of the first two lines accommodate the pentameter structure as a whole.
This almost anti-metrical beginning suggests that Shakespeare is intentionally drawing on the prose cadences of the Book of Common Prayer, from which he draws much of his vocabulary for the sonnet ("impediment," etc.). In a way, you could say that it is his fault the poem has become a wedding cliche.
Helping to counter this is the suitably grim third quatrain, which treats the mutability topos (all things decay in time, etc.) like an extreme sport. Check the "compass" of Time's sickle: that bitch is swinging his blade like a movie serial killer. Those "rosy lips and cheeks" just got a whole lot rosier, yo, lying there in a big dismembered heap. English Literature is not pretty.
(I omit commentary on the second quatrain for the sake of space, and because I have nothing terribly original to say about it, other than that I can't help but hear "every wandering bark" as an unintentional index of Love's indiscriminate nature: like, "any port in a storm" or something.)
The couplet is about as unstable a proposition as one could hope to find. Someone please tell me how the "if/then" logic is supposed to work in it. Why would the demonstration of the untruth of the proposition that true love is eminently constant enforce the conclusion that the author had never written, or that no one in history had ever been in love (or, depending on how you construe the grammar, that the author had never loved any man)? Am I missing something obvious, or is this just desperately giddy rhetoric?