How does Shakespeare's use of figurative language create contrasting themes across sonnet 18 and sonnet 147?

In "Sonnet 18'', the speaker describes how the person he addresses is more sweet, temperate and fair than the beauty he sees in nature. He even notes how the sun is sometimes dim and how nature’s beauty is sporadic. And in "Sonnet 147,'' the speaker realizes he is in over his head in love. He compares love to a disease, a fever that turns him mad and from which he cannot escape.