Ironically, Shakespeare both glorifies and immortalizes the "young man" in "Sonnet 18" through the poetic verse itself. This echos the theme "Sonnet 18" which is eternal beauty through verse. This is known as possibly "the" best of Shakespeare's sonnets beginning with the immortal line, "Shall I compare thee to a summer's day?" It is one of the many sonnets written to the elusive "young man," who everyone is always wondering about.
In regards to exactly "how" Shakespeare goes about glorifying and immortalizing, that answer is closely related to theme. At the beginning of the poem, the young man is said to be "more lovely and more temperate" than a day in the season of summer. The speaker then immediately gives many examples of negative summer day issues such as wind, longevity, and heat. By the end of the poem, it is clear that the reason why "thy eternal summer shall not fade" is precisely because Shakespeare has written this poem. Published poetic verse truly does lend immortality to the subject it's written about. Therefore, the theme of "Sonnet 18" is summed up the best in the last two lines:
So long as men can breathe, or eyes can see, / So long lives this, and this gives life to thee.
The "eternal beauty" of Shakespeare's friend has been immortalized by Shakespeare in sonnet number 18, by emotions through which he portrayed the beauty and the usage of metaphor.
While reading sonnet number 18, any reader will form an image of Shakespeare's friend in his subconscious mind. The intense feeling by which Shakespeare presents his feeling and portrayed the beauty of his friend is remarkable.
The sonnet begins with the comparsion of Shakeapeare's friend with the "summer's day" and then he concluded saying that his friend is more lovely and temperate than summer's day, further in the sonnet he showed the superiority of his friend "eternal beauty" over the beauty of the nature.
In the ending couplet, Shakespeare consciously knew that this sonnet will immortalize the beauty of his friend, because all the coming generation will keep forming the glorious image of his friend while reading this sonnet. "so long as men can breathe or eyes can see, so long lives this, and this gives life to thee"