Why did William Shakespeare write sonnet 18?

William Shakespeare did not write Sonnet 18. It was composed by another English Renaissance poet and dramatist, Henry Constable during the late 16th century before 1592. There seems to be some disagreement between scholars regarding Constable as its author with George Chapman proposed as alternative writer of the piece along with Shakespeare himself. Sonnet 18 consists of fourteen lines which follow the format, iambic pentameter and ABAB CDCD EFEF GG quatrain and couplet rhyming.