What is the purpose of these lines? to complain about the intense heat and humidity of summer
to persuade a loved one to visit during a warm season
to express love by likening a loved one to a nice day
to argue that hot weather is more pleasant than cold weather

Here are the lines:  Shall I compare thee to a summer's day? Thou art more lovely and more temperate: Rough winds do shake the darling buds of May, And summer's lease hath all too short a date

to express love by likening a loved one to a nice day

Explanation:

Comparisons are most times used in literature to help people have better visualization and appreciation of a subject being discussed by a speaker. In these lines from Shakespeare's Sonnet 18, he likens his loved one to a summer's day. Just as a summer day was bright and temperate, so was his lover lovely and gentle. He also likens his lover to a beautiful bud that remained unshaken despite the unfavorable winds.

i've read it so how can i ?