Shakespeare, the sonneteer of "Shall I compare thee," begins his sonnet with a question that he will proceed to answer in the negative. There are two key lines that reveal this and show that a comparison between "a summer's day" and the one who is "more lovely and more temperate" than a summer's day is a negative, or, if you will, an inappropriate comparison. The first is “Thou art more lovely” and the second is “But thy eternal summer.”
The sonneteer explains that while "rough winds" may "shake the ... buds of May," her beauty is unmarred by either rough winds or the sun, that "too hot eye of heaven," that is also sometimes dimmed in "his gold complexion."
The sonneteer further explains that the beauty of the beloved is not like the beauty of summer and that even Death won’t rob her beauty because the poet is perfecting her in "eternal lines" where "in eternal lines to time" she is, to coin a word, eternalized. He promises that as "long as men can breathe or eyes can see," she will live because this sonnet "gives life" to her.
It is in these ways that a comparison between his beloved and "a summer's day" is a negative, or inappropriate, one: (1) Summer suffers declining beauty even as it revels in its brightest day, while she will never fade as summer does nor ever die because (2) her beauty is written in a sonnet and the sonnet will live and she with it.