Ah, sonnet 18. I think you are talking about one about a summer's day and a young adult. Let me give it a whirl.
Shakespeare wants to compare a summer's day to a young person. It sounds nice in the beginning but then he begins to discuss what this might really mean.
Even though a summer day might be nice, it might also be unpredictable. Some days are too hot. Some days are too windy for one's liking. Shakespeare notices that young people might be the same way that nature is, hence the weather imagery. On some days, teen can be overaring or inconsistent.
In the end, Shakespeare would like to see that this young person become something more constant and immortal like this sonnet.