Shakespeare plays with conventional love imagery in this poet. As he does in Sonnet 130, he shows that the traditional "natural" metaphors fail to adequately reflect the true subject of the poem. He could compare her to a "summer's day"--that is a woman in full bloom, so to speak--but the metaphor breaks down. The inadequacy of this metaphor is clear: sometimes it gets "too hot" under the eye of heaven; at other times, the sun's complexion is dimmed. Both of these metaphors suggest extreme hot or cold and do not show her as she truly is--temperate. In addition, he knows that earthly beauty is subject to the elements and will fade eventually. This also does not satisfy him. The "but" after the first octave signals the turn in his argument. He has found a way to capture her fully formed beauty (eternal summer)for all time. He will write of her, and in his words, she shall live forever.
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.