It's almost like a parody to other sonnets in that it is sarcastic, and tells the truth of how the speaker really feels about his love. The speaker is thinking of every beautiful thing, comparing it to his love but in a negative way. Like when it says that he loves to hear her speak, but the sound of music is much more beautiful than the sound of her voice.
In the end, it summarizes in saying that while their lover's love is as rare as heaven, their comparisons to these beautiful things will always stand as false considering how negatively the speaker talks of his love.