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.
This poetry of the beloved’s beauty is written in “lines” so that he can be remembered for eternity. “Eternal lines” is also a play on words, in the sense of  bloodlines: he lives on through the poet’s work as we live on through our heirs and the children we procreate. 
 The power of poetry (or art) in opposition to the power of time is a  recurring theme in Shakespeare’s sonnets.
Sonnet 18