Shall I compare thee to a summer's day?"Sonnet 18," 1–4
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 . . . .
The speaker of this poem compares the best of days to a paragon of youth; and the paragon, probably a young nobleman, wins the competition. The speaker finds him lovelier and more "temperate" (mild) than the often punishing days of summer. (The poet changes his mind about this in later sonnets.) And while summer and its beauties pass away—its "lease" (tenure) has too short a "date" (duration)—the young man's good looks will endure: "thy eternal summer shall not fade" (line 9). But this isn't because the youth has lots of Retin-A stored away. His beauty will endure forever because this sonnet will endure' forever [compare GILDED MONUMENTS].
We treat "Shall I compare thee to a summer's day" as a compliment to the lover whose glories are supposed to match summer's. The poet's point is different: the lover isn't measured against summer, summer is measured against the lover (as he's captured in poetry) and is found lacking.