In Sonnet 18 Shakespeare says that comparing his love to a summer's day is a bit pointless. Why? In a nut shell because summer will eventually fade away and come to an end. Shakespeare writes that her beauty and awesomeness will be an eternal summer. It will not fade and will forever be beautiful. I'd like to say that is because she has the superpower of immortality, but sadly that is not the case. Her beauty will never fade because Shakespeare wrote about it. His writings will live on forever, and as long as men can read, they will always be able to read about how beautiful she is. She will essentially live on forever through his poetry. It's quite romantic. It also shows that Shakespeare has a fairly high opinion of his poetry, too.
"When in eternal lines to time thou grow’st;
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee."