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.