Sonnet 18 begins with the narrator asking if he should compare the subject, which we will assume is a woman, to a summer's day. Because Shakespeare asks if he should make this comparison implies that it is arbitrary. Shakespeare is asserting that Sonnet 18 could quite as easily be about the woman's comparison to anything beautiful because she is more dazzling, or "more lovely", as Shakespeare asserts in the second line when he begins his comparison, than any other beauteous object or concept in the natural world. Thus, Shakespeare's decision to compare her to a summer's day is meant as a joke and each element he mentions, to which he compares the subject, becomes extensions of that jest. In this way, Sonnet 18 is more playful than a superficial reading may suggest.
Sonnet 18 contains the elements of beauty, weather, warmth, and duration with which Shakespeare compares his lover. As is explained above, he begins by stating his love is more beautiful and goes on to elaborate why he finds her to be this way. Firstly, Shakespeare finds her temperament to be more pleasing than that of summer. He claims "Rough winds do shake the darling buds of May," and "Sometime too hot the eye of heaven shines," or that summer sometimes brings an unwanted climate in which beauty is endangered, as represented by the shaking of flowers, and that it is sometimes too intense, as represented by the heat of the sun. Contrastingly, the subject of Sonnet 18 is "more temperate" and while embodying the favorable aspects of summer, does not bring about any sort of unpleasant characteristic summer is known to bestow. Furthermore, Shakespeare claims "And summer's lease hath all too short a date." or that summer is fleeting, implying that the subject of Sonnet 18 is constantly beautiful.
Extending upon the idea that summer is only temporary whereas the subject of Sonnet 18 is not, Shakespeare writes "And often is his gold complexion dimmed;" and "And every fair from fair sometime declines," or...