Shall I compare thee to a summer's day ; Thou art more lovely. explain!

These are the two opening lines of Shakespeare's Sonnet -XVIII. Here the poet highlights the quality of his friend by comparing him with a summer's day.

The poet wants to glorify the beauty and quality of the person to whom he has addressed this sonnet. It is generally thought that the figure whom Shakespeare addresses in sonnet XVIII is his friend the Earl of Southampton. Whoever may be the person the poet concentrates on the physical and spiritual aspects of that person. At first the poet compares him with the beauty of a summer’s day. It is worth noting that summer in England is not like the summer in Bangladesh. 

In England summer is pleasant and weather condition is usually nice. By comparing  his friend with a summer’s day the poet wants to indicate the brightness, beauty and charm of his friend. But the poet goes farther to say that his friend is more beautiful and more temperate than a summer’s day. According to the poet a summer’s day has some defects or shortcomings because seasonal course suffers from the whim of nature. For example, the sweet buds of May are torn by rough winds. The poet wants to say that the beauty of his friend supersedes the beauty of a summer’s day.