The couplet reverses the ideas stated in the first three quatrains.
The first three quatrains all insult his love. They say her breath stinks, music is more pleasing than her voice, her hair is like wires...etc. Even though he goes through all these characteristics of his love and says how they are all lacking in some way, at the end of the sonnet he declares his love for her. Throughout the poem the reader wonders if the speaker actually loves the woman he is describing, but in the last couplet his love for her is unquestioning.