Nature positively affects the author's state of mind in a variety of ways.
First, the author experiences much of nature starting with the sky by imagining himself a cloud. This point of view gives him the opportunity to see how vast the beautiful and happy flowers are. It seems to him that at a glance his eye can capture tens of thousands of them. Also, he considers the daffodils in comparison to the sparkling waves of the ocean. He seems to say that the daffodils are happier than the waves.
Second, the author uses word choice to express how the daffodils make him feel: pleasure, jocund, glee, and gay.
Third, he gives the daffodils action, the ability to dance. This could also be considered personification. Dancing is an act that occurs out of celebration, not depression.
Finally, the author reflects later upon this image and he can re-create the image in his head, especially when he's in an empty mood. This image is one he wishes to remember and upon remembering it, his mood is brightened:
For oft, when on my couch I lie
In vacant or in pensive mood,
They flash upon that inward eye
Which is the bliss of solitude;
And then my heart with pleasure fills,
And dances with the daffodils.
P.S. I noticed that you had several questions in your opportunity to provide further detail to explain your original question. As editors, we are only to answer one question at a time, so you can repost those in separate questions if you like.
Nature can be very complicated expecially when it deals with the state of mind. Nature is the material world and its phenomena.