What connection is the poet trying to establish between nature and himself in "Lines Written in Early Spring"?

The speaker sees himself as an intrinsic part of nature, no less than the birds and the periwinkles and the "budding twigs" that give him such enormous pleasure. By identifying himself so closely with the natural world he's consciously setting himself apart from humanity, with which he's become so thoroughly disillusioned.
Everything the speaker sees around him is part of what he calls "Nature's holy plan." This shows us that he doesn't regard the natural world as just something pretty to look at but also as a powerful, almost quasi-divine force with its own internal dynamic. That being so, it causes the speaker sadness to see how man has made such an almighty mess of his relations with his fellow men.
Nature is characterized by the close bonds between every living thing, and the speaker for one feels those bonds more keenly than anyone. But man has chosen not to follow the example of nature, and instead, his relations with his fellow men are so often marked by mutual antagonism.
https://www.poetryfoundation.org/poems/51001/lines-written-in-early-spring