The speaker personifies wind in the third line of “Sonnet 18.”
Often referred to by its first line as “Shall I Compare Thee to a Summer’s Day,” Shakespeare’s “Sonnet 18” compares a woman to a summer day. Like a summer day, a woman’s beauty is not necessarily going to last.
Personification is describing something nonhuman with a human quality. In this case, the wind is described as being “rough” in the way that a person would, intentionally shaking the delicate buds.
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date: (lines 3-4)
Too soon, the summer’s day with end, just as a woman’s youth will. Each is beautiful and wonderful, and worthy of celebration. However, part of the reason is the fact that neither will last. We appreciate them for the brief time we have them.
Personification allows Shakespeare to add emotion to the poem. The image of a mean wind messing with the helpless flowers will stick in our heads, and we will feel sorry that neither the summer day nor the woman's youth can last forever.