Key to understanding this excellent (and rather amusing) sonnet is understanding the way that Shakespeare uses it to ridicule the fashionable and rather over-the-top metaphors other contemporary poets were utilising to describe the women they wrote about. Shakespeare therefore pokes fun at such conceits and argues for a more realistic approach to describing his beloved.
This is why the poem consists in a large number of negatives and surprises the reader. For those expecting a conventional love poem, the way that the poet describes his beloved's lack of beauty comes as a bit of a shock:
My mistress' eyes are nothign like the sun,
Coral is far more red than her lips' red.
If snow be white, why then her breasts are dun,
If hairs be wires, black wires grown on her head.
And yet, in spite of this, the tone changes in lines 9 and 13-14 when the speaker assures us that he loves his beloved in spite of, and perhaps because of, these imperfections:
And yet, by Heaven, I think my love as rare
As any she belied with false compare.
Shakespeare therefore seems to be using this poem to do two things: firstly, to mock trite conceits and exaggerated descriptions, and secondly to affirm that love exists between real, mortal men and women, and that it is just as valid.