When Shakespeare argues that his beloved is more lovely than a summer's day in Sonnet 18, one thing he is thinking of is how short lived summer is. He points out that "summer's lease hath all too short a date," meaning that summer only "leases," or inhabits earth for a short period of time. He argues instead that his beloved's beauty "shall not fade," nor shall she loose her beauty entirely: "Nor lose possession of that fair thou owest."
Another reason he says his love is more beautiful than a summer day is that the summer whether can be harsh, making nature look less beautiful than it does otherwise. In his imagery, he points out that the summer sun gets too hot, making things look dim and brown. Since summer can make things look brown, of course his beloved is more lovely than a summer day, because she does not look coarse and brown. Also, beyond the summer season being short lived, his imagery points to the fact that beauty within the summer season is also short lived. Summer is the season in which colors in nature look richest and brightest; however, these rich bright colors are short lived within the summer season because soon the heat from the harsh sun makes things fade. Hence, since beauty seen at summer time is short lived, his beloved is more beautiful than a summer day. Therefore, not only is his love more beautiful than a summer day because she is not course and brown, her beauty lasts longer than nature's beauty within the summer season.
In Sonnet 18, Shakespeare is comparing a beautiful woman to a day in the summer. He goes on to tell us that although summer is a season that most of us enjoy, the time will come that summer will end. He also gives us some things about summer that we may not like. Sometimes the sun is too strong; sometimes the wind blows very strong, especially before a thunderstorm, and the spring flowers are destroyed. Sometimes there are clouds instead of sun (especially in England). and, eventually, summer fades to fall and winter. Shakespeare is saying that his loved one's beauty is more stable than summer; her beauty will never fade because of her "eternal summer (Line 9, "Sonnet 18")." It may be that her unending season is a reference to a painting, or this poem, of her. In this way, the artwork will always show her in her "summer" of beauty, as long as people can see it.
The couplet refers to the art that has been chosen, in this case, this sonnet. Even though Shakespeare has been dead for quite a while, we still know that he loved this person. Why? Because "this" still lives, and men still breathe and have eyes to read it. In the same way, all pieces of art are an eternal testament to the artist's emotions about the subject matter, as long as that art is not destroyed.