The first line of Shakespeare's Sonnet 18 appears to be a question:
Shall I compare thee to a summer's day?
Shakespeare doesn't ask, "May I," or "Can I," or "Would you mind if I," nor in any way does he ask for permission or even for acquiescence from the person to whom the poem is addressed, to compare them to a summer's day. Shakespeare instead employs a rhetorical question, which effectively turns the question into a statement.
In asking, "Shall I compare thee to a summer's day?" Shakespeare has already done it. He's already compared the person to whom the poem is addressed to a summer's day, at least in the abstract. He continues the comparison through the next seven lines by referring to and comparing the person to whom the poem is addressed to certain, specific aspects of a summer's day.
"Thou art more lovely and more temperate," Shakespeare writes, then lists the ways in which the person to whom the poem is addressed isn't like a summer's day.
Nothing that Shakespeare writes about in these eight lines expresses the true theme of the poem. The first eight lines are simply the preamble to the next four lines, in which Shakespeare moves a little closer to the theme.
The ninth line, "But thy eternal summer shall not fade," intentionally leads the person to whom the poem is addressed to ask the question, "Why not? Why won't my eternal summer fade?"
Three lines later, Shakespeare answers the question: "When in eternal lines to time thou grow'st." When someone writes a poem ("eternal lines") about you, Shakespeare says, speaking to the person to whom the poem is addressed, then your "eternal summer," your eternal youth, your eternal beauty can never fade.
In the concluding couplet, Shakespeare finally gets to the theme of the poem:
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.
The poem isn't about a summer's day or even about the person to whom the poem is addressed. The poem is about Shakespeare himself. Through "this," through my poem, Shakespeare says to the person to whom the poem is addressed, you are immortalized; and you, your youth, and your beauty will live forever.