How does Shakespeare develop the thought of his "Sonnet 18"

“Sonnet-18” occurs in a long sequence of 154 sonnets composed by Shakespeare, the best-known sonneteer of English literature. It is one of & group of 126 sonnets that overwhelmingly appreciate the beauty of a “fair youth”, a friend of the persona, in most warm language. The poem opens with a majestic image of the “summer’s day” and soon passes on to several other images. The latter images are used to mean the negative qualities of the “summer’s day” and suggest the rarity of the “fair youth”. Thus, the persona’s emotional choice for a unique human form has been rationalised through a series of images leading to a very bold assertion of the friend’s immortality. 

The first line poses a question: “Shall I compare thee to a summer’s day?” Here the desire behind the comparison is obviously to emphasise the charm of the friend. A summer day in England is famous for its sunshine, beauty and charm. The image of the “summer’s day” has been used here to indicate the brightness, beauty and charm of the “fair youth”. To the speaker, his friend first appears as radiant, mirthful and charming as a day in summer. But soon he corrects himself saying that his friend is “more lovely and more temperate.” The repeated use of the word “more” implies glorification of the friend. Again, glorification is there in the doubt implied in the question. The doubt is whether “a summer’s day” has all the qualities that the friend has. The first image, thus, glorifies the “fair youth”. 

The doubt has been rationally explained in the lines that follow. The persona now goes on pointing out the better qualities of his friend by contrasting them with the worse qualities of the “summer’ day” through a series of images. There is in summer “rough wind” that destroys “the darling buds of May”. Moreover, it is transient. The image of the rough wind is followed by the image of the scorching, hot sun. Even the golder. complexion of the summer suffers from uncertainties as, sometimes, a piece of cloud or an eclipse may change it. 

But the friend, the speaker believes, enjoys eternal youth and charm. His beauty will never be faded because he is presented in the poetic lines which will never die. The beautiful image of the youth made in this poem will continue to live as long as human beings live on earth and read this poem. It is, therefore, clear that the theme of immortality has been developed in this poem through the images of summer, rough wind, flowers of spring, sun, heat, cloud and eclipse. They have been used either to compare or to contrast. In. both the cases, the intention is to glorify the friend and his permanent positive qualities. He has been immortalised with the immortal lines of the poem.