In this sonnet Shakespeare is parodying the conventional comparisons of traditional love poetry (and specifically, of Petrarchan sonnets). The speaker says that of course his "mistress' eyes are nothing like the sun," nor her lips as red as coral. A traditional love poem might use such clichéd similes to describe the beauty of a woman, but the speaker in this poem, for the first twelve lines at least, seems to refuse to do so.
The poem is structured as a series of comparisons, each one formed as a simile, and most of the comparisons throughout the poem are physical. The speaker thus describes his mistress' eyes, lips, breasts, and cheeks. Bearing in mind that the poem is intended as a parody, the preponderance of physical comparisons is perhaps intended to highlight how superficial those comparisons are. Real love, after all, should transcend the physical.
The first twelve lines of the poem are full of comparisons which seem to suggest that the speaker is not very fond of his mistress,...
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