The majority of sonnets structurally include what is called a "turn," or a shift in focus or thought. Where this turn occurs in each sonnet of course varies, but what is interesting about this particular sonnet is that it only occurs in the very last rhyming couplet. Having spent the three quatrains establishing the main theme of the sonnet, talking about the constancy of love in the face of the passage of time and the fading of beauty, the turn then serves to reinforce what the speaker has been saying by a shift to a far more emphatic, direct tone and the use of irony:
If this be error and upon me proved,
I never writ, nor no man ever loved.
Thus having established his central argument in the main body of the poem, the poet delays the turn in this sonnet until the last rhyming couplet, and changes the tone and introduces irony to highlight the truth of what he has been arguing - true love does not change, and if he is wrong, then he is no writer and nobody has ever loved.