The contrast created in this poem is through the presentation of nature as coexisting in glorious unity and beauty. Various sights that the speaker looks at seems ample testament to this fact. However, what makes the speaker sad when looking at such beauty is the way in which he compares "What man has made of man" to the sight before him. Whenever he sees yet another example of the beauty in nature, it only seems to remind him of the terrible way in which we are so obviously unable to coexist or function in our lives with pleasure and peaceful unity. Consider how this contrast is presented in the following stanza:
The birds around me hopped and played,
Their thoughts I cannot measure:--
But the least motion which they made
It seemed a thrill of pleasure.
If Nature did indeed "link the human soul" to her creation, it is a terrible thing to compare nature and ourselves as a species and to think of Nature's "holy plan" as a result. We seem to have fallen so short of our potential in our relationship with nature, and this causes the speaker to "lament" as he sees the beauty of nature and all the potential that his species has missed out on.