Central to understanding this brilliant sonnet is the way that Shakespeare is actually poking fun at the somewhat hyperbolic and exaggerated descriptions of the idealised female of conventional love poems. Shakespeare deliberately eschews such high-blown and innaccurate description, indicated by the series of negatives in the poem. In addition, the speaker seems to deliberately emphasise the less than divine aspects of his lover's character, focusing on her bad breath, her wiry hair and her normal eyes:
And in some perfumes is there more delight
Than in the breath that from my mistress reeks...
Such descriptions give this sonnet a humorous honesty that deliberately undercuts the exaggerated descriptions of other love poems, which present the object of the poem as almost a "goddess." However, in spite of this frankness, Shakespeare uses this kind of description to suggest that love exists between real and not idealised characters, and that this love is far more genuine because of this honesty. Note how the closing couplet supports this theme:
And yet, by Heaven, I think my love as rare
As any she belied with false compare.
Shakespeare is therefore playing with our expectations of a love poem. The frank descriptions do not indicate a lack of love or regard, rather they are used to emphasise the far more genuine nature of the love that exists between them than the ridiculously exaggerated descriptions provided in most love poems of the time. By criticising his beloved, he is actually only reinforcing his love for her.