Taking the title in consideration, you soon can find the alliteration of letter "m". "My" is the most important word of the title. Instead, "Mistress" does not only mean "lady", it indicates a lady who has power on the poet.
Considering the layout, you can understand this sonnet follows the Elizabethan model, and according to this convention the reader should suddenly find the personal and sentimental situation in the three quatrains.
Right from the start, the reader finds a statement: the speaking voice says that there's nothing better than the sun. The poet is comparing his mistress to the sun, the coral and to the snow. Thus, the first quatrain rely on a comparison between his mistress' body and elements taken from nature.
This sonnet is strange because people write sonnets to appreciate someone or something. Composing this sonnet, Shakespeare turned the courtly love convention upside down.
Nature is much better than his mistress. The first quatrain is the introductory quatrain, where the poet introduces a comparison, in which the mistress is the loser because, for example, her eyes are not as beautiful as the sun. In the second quatrain repeats the same comparison and once more the lady is the loser because her cheeks are less red than damasked roses and he also adds that profumes are much better than her breath. The first two quatrains expresses the situation and the third seems to introduce something different because the poet introduces something completely opposite from the courtly love traditions. Shakespeare says that he likes listening to his lover speech even if he is completely aware that music is much more pleasant to listen to.
In the ending line of the third quatrain the speaking voice confesses he has never seen a Goddess walk but he knows his lady "treads on the ground".
The three quatrains that generally illustrate the difficult aspects of the speaker are meant to anticipate the conclusion that is expressed in the rhyming couplet, so that they also respond to a climatic effect: the one of the rhyming couplet, where the poet confesses that he loves his lady all the same,even if she is not the perfect lady of courtly love poetry.
Shakespeare choice is meant to be a parody of the conventional love poetry. The poet is aware that love is something that goes beyond simple physical aspect and implies more.