The entire basis of this excellent and famous Sonnet by Shakespeare is the way that he compares his beloved, the object of his affections, to a "summer's day" and finds the beloved superior in every sense. Thus the poem begins with the famous comparison, and then the speaker of the poem finds summer wanting in every aspect in comparison with the beloved:
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.
Thus it is that in the speaker's mind, the beloved is superior to a summer's day, because he is more "lovely" and more "temperate," because a summer's day is characterised by brevity and the "rough winds" which detracts from the overall beauty of such a day. In every aspect, the beloved is superior to such a comparison.