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;
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 wanderest 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.
Personified “Death” will never boast that he has the young man because the poem has immortalised him. 
 The capitalisation enhances that idea of death, a concrete being that was ever-present at the time; a source of terror.  Lives were short and mortality high amongst even young people.  The speaker is saying that even Death, an unstoppable force, cannot take his lover.  So the  metaphor  “wanderest in his shade” is a  euphemism , appropriate for this gentle poem.
Sonnet 18