This suspicion becomes even more ironically clear as we read further. As we progress with the analysis of the protagonist's description of his love we find even more apparently negative comparisons. For example, he states that that in comparison to perfumes his "mistress reeks" and that music has a much more "pleasing sound" than her voice. He also states that she is no goddess in the lines,

I grant I never saw a goddess go;

My mistress, when she walks, treads on the ground

However in the final couplet of the sonnet there is a dramatic change of tone and a radical change in our perception of the loved one. The...

The second last line begins with an ironic reversal of all that has been apparently stated about his love and is prefigured by the phrase " And yet…." The poet goes on to state that his love is "rare," which implies uniqueness and an individuality that "belies" all false comparisons. In other words, all comparisons are false and do not live up to the actual reality of his love.
This poem is extremely ironic in the way that it presents metaphors and comparative images that seem on the surface to be negative and disparaging. In the concluding couplet however this impression is reversed. In the final analysis the image of the loved one transcends all comparison and cannot be reduced to any trite or conventional metaphor. This implies that the figure of his love is unique and that her beauty and wonder is beyond any easy comparison or description -- which is possibly the highest praise.