When the poet asks whether he should compare his beloved to a summer's day, the word "compare" connotes observing both similarities and differences. The poet goes on to note the differences between his beloved and a summer's day. First of all, he notes that she is "more lovely." Immediately this brings to mind physical beauty; a summer day has that with its greenery, blue sky, and flowers. Yet when we say a day is "lovely," we often refer to more than its physical appearance. Summer days have a "lovely" quality that includes comfort, peace, and ease. Thus the poet believes his lady creates more emotional comfort than a summer day.
Second, the poet states his lady is "more temperate" than a summer day. This word has a dual meaning: in one sense, it relates to mild temperature; in another, it refers to self-restraint and constancy. With this pun the poet points out that his lady is more dependable than a summer day. He goes on to say that "rough winds" can disturb an otherwise fine day, and summer often fades too quickly.
Third, developing the thought of summer's lack of reliability, the poet notes that its gold eventually dims and its fairness "declines." His lady, he asserts, will "not fade" because his love, immortalized by this sonnet, will keep her as "eternal summer."
The idea behind the sonnet is that all the good things that one associates with summer apply to the poet's beloved, but that she surpasses that season because of her own loveliness and constancy and because of the immortality she receives from his love.