Sonnet 18 begins with asking the lady if he shall compare her to a summer's day--sunny, bright, carefree, full of all that is an extension of spring--but then tells her why she is nothing like a summer's day. She is better--"more lovely and more temperate". She is not rough like the winds that shake the flowering buds of May, nor is she as short tempered as the summer season. She is even-tempered and lovely for a much longer period of time...possibly forever in his heart and mind. She will not even succumb to Death since he has written about her in this poem. She will indeed last--as will his love and admiration for her--as long as the lines of the poem do.
This fourteen-line poem begins with a straightforward question in the first person, addressed to the object of the poet’s attention: “Shall I compare thee to a summer’s day?” After a direct answer, “Thou art more lovely and more temperate,” the next seven lines of the poem develop the comparison with a series of objections to a summer day. For one example, he thinks that Summer and the May winds shake the buds. In lines 7 and 8, the poet summarizes his objections to the summer day by asserting that everything that is fair will be “untrimmed,” either by chance or by a natural process. The most obvious meaning here is that everything that summer produces will become less beautiful over time. The last six lines indicate that the person about whom Shakespeare is writing, will never be forgotten or fade, because she will be immortalized in the Sonnet.
Basically, in SONNET XVIII, Shakespeare is talking about someone he loves. He is doing as he states in the first line, comparing this person to a summer’s day…Summers beauty may come and go, but this persons beauty will never fade away…This sonnet represents that persons life; as long as people read or see this poem, that person shall live on forever.
When Shakespeare wrote a poem about a girl, it’s the same as a singer writing a love song---a poet writing a love poem to this person trying to impress her (most likely) and technically trying to carry on her legacy and beauty through his words.
the poet compares a summer day to his beloved