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
Summer is short, despite the speaker’s wish that it last longer. Throughout the sonnets Shakespeare frames his love in explicitly or implicitly practical or concrete terms. But here Shakespeare has asked if he should compare the subject to a “Summer’s” day, but has refused and instead told him that the Fair Youth is too beautiful and beyond comparison, beyond fading. 
 So what does “And Summer’s lease hath all too short a date” mean? Shakespeare has already told him that he is beyond comparison; he is better even than Summer with its short “lease”, or timespan, such is the depth of speaker’s love.
Sonnet 18