How does Shakespeare glorify his friend in Sonnet 18?

Shakespeare glorifies his friend in two ways, as evidenced in the structure of the poem. In the first six lines, he compares his friend to a summer day and explains how summer is less perfect and desirable, which glorifies the nature of his friend:

Shall I compare thee to a summer's day?

Thou art more lovely and more temperate:

Rough winds do shake the darling buds of May,

And summer's lease hath all too short a date:

Sometime too hot the eye of heaven shines,

And often is his gold complexion dimm'd;

Summer is too short a season, its days subject to "rough winds." Summer days are sometimes "too hot" and frequently the sun is "dimm'd" by clouds or inclement weather, it suggests. Shakespeare's friend, however, is "more lovely and more temperate." Metaphorically, then, we could infer that Shakespeare's friend is a pleasant person (lovely) with a "temperate" nature, reliable and trustworthy, one not given to emotional outbursts or dark moods.

In the remainder of the sonnet, Shakespeare emphasizes that nature is transitory, but his friend will be immortal, living within the lines of the sonnet as long as the sonnet can be read:

And every fair from fair sometime declines,

By chance or nature's changing course untrimm'd;

But thy eternal summer shall not fade

Nor lose possession of that fair thou owest;

Nor shall Death brag thou wander'st in his shade,

When in eternal lines to time thou growest:

So long as men can breathe or eyes can see,

So long lives this and this gives life to thee.

Everything "fair" in nature "declines." Nature's course is one of change. His friend, however, "shall not fade" or become less fair, or even die. Shakespeare glorifies his friend by preserving his life in Sonnet 18:

So long as men can breathe or eyes can see,

So long lives this [sonnet] and this [sonnet] gives life to thee.

Perhaps Shakespeare was correct. More than four centuries later, we are still reading Sonnet 18 and talking about his friend.