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:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd;

 Spring produces new flowers and leaves on trees; however, they are blown away by the strong winds that Summer brings. 
 So the transition from Spring to Summer is a violent one. Shakespeare contrasts this turbulence with the constant, “eternal” nature of the lover’s beauty, suggesting that it is more beautiful than any passing season. 
 Shakespeare uses the word ‘Rough’ to describe the winds so that he can convey his feelings to the audience. The use of an aggressive word such as ‘Rough’ allows the audience to understand that his love for her/him is passionate rather than gentle. 
 Rhythmically, the first two words of this line are  spondees , that is, they are equally emphasised, with long vowels and matching stress, disrupting the regularity of the  iambic pentameter  metrical rhythm.
Sonnet 18