Why do you think Shakespeare begins "Sonnet 18" with a question?

By opening the poem with a rhetorical question, the speaker is almost teasing his lover. He seems to be saying "Shall I compare thee to a summer's day? Maybe I should, but then again, maybe I shouldn't. Who knows?" Beneath the mild joshing, however, is a revealing statement of the speaker's uncertainty. He genuinely doesn't know whether or not it would be appropriate to compare his lover to a summer's day.
That's not because he thinks that the object of his affection isn't lovely; quite the opposite. For as the poem progresses, it's clear that the speaker finds his lover so much lovelier—not to mention more temperate—than a summer's day. It's simply that the speaker realizes that, however beautiful a summer's day may be, it will never adequately be able to capture the eternal beauty encapsulated by his lover's soul.
https://www.poetryfoundation.org/poems/45087/sonnet-18-shall-i-compare-thee-to-a-summers-day