The speaker of Shakespeare's "Sonnet 18" compares his loved one and a summer day and finds the summer day to be lacking. In the first four lines of the poem the speaker identifies three shortcomings of a summer day.
They say that it is too severe. The speaker's love is more "temperate," or mild and modest. This is a criticism of a summer day's harsh temperature and unpredictable changes in weather. Perhaps related is that, on the whole, the speaker finds the subject of his heart's affection to be more "lovely" than a summer's day.
Additional, he says that a summer's day is rough with vulnerable living things. Line three takes issue with a summer's day in that its "rough winds do shake the darling buds of May." Note not only that the winds are rough, but the speaker observes that they are abusing the sweet, vulnerable, "darling" buds of May. Ostensibly, he is arguing that his love would never be so rough with such a gentle living thing as a bud.
He also says that it is too brief. The speaker faults a summer's day (or rather, the entire summer season) as being all too short: its "lease hath all too short a date." We can infer that the speaker's love is more enduring and present, whereas the summer is a series of fleeting sensations. Both a summer's day and the summer season itself come and go too quickly to be enjoyed.