In "Sonnet 18," Shakespeare wishes to convey the beauty of his lover through an extended metaphor in which he compares her loveliness to a summer day. Not all Shakespeare sonnets are created equally--in this particular sonnet, he chooses to use more traditional imagery and metaphors through nature. In "Sonnet 18," the speaker's lover is cast in a fair and favorable light.
"Sonnet 2" contrasts sharply with the ideas and imagery from "Sonnet 18." Rather than writing of the appealing traits of the speaker's beloved, Shakespeare uses a different season, winter, to show the ravages of time on "beauty's field" (2). In this poem, the speaker's loved one's aging features reinforces the theme that beauty is fleeting and cannot last.
Both sonnets speak to themes of beauty, nature, and timeless love, and Shakespeare reveals different aspects of these themes through contrasting imagery and figurative language.