He thinks that most happiness comes from nature, but he is sad because he does not think that people realize that. He uses sunshine and clouds as a metaphor.
The function of nature in a single poem is that nature is supposed to be representing human life. Most of the seasons and weather changes are seen as metaphors in a human's life.
Both authors are similar as they use poetry as a form of expression. They both use forces of nature to do this.
Happiness is an emotion that almost anyone is attracted to. Everyone, for the most part, strives to be happy.