Walt Whitman's "Song of Myself" fits within the American Romantic movement. As such, it shares the reverence for nature one finds in all Romantics. Like his intellectual hero Emerson, Whitman believes that nature has more to teach humans than any creed or constitution, and the poem is a celebration of the individual and collective self in a state of nature and in a society that honors the teachings of nature:
Creeds and schools in abeyance,
Retiring back a while sufficed at what they are, but never forgotten,
I harbor for good or bad, I permit to speak at every hazard,
Nature without check with original energy.
We find numerous lines referring to the natural realm, and each suggest that humans find their best selves in communion with the natural world.
Being one with nature, even to becoming physically what we consider not human but merely nature, is to Whitman a comfort in which he exults. His great continuance is possible because we are natural beings who need not shun our origins as people of earth.
In particular, the passages in which we find the word "grass" speak to this privileging of the natural world as the pathway to deeper wisdom and to greater pleasure. The poem becomes in these passages an invitation to set loose the societal constraints that inhibit the natural self.
Whitman ends his poem with a meditation on the circle of life comparable to Hamlet's much darker musings (and to that which we find in the Lion King). As creatures of nature, all of us will eventually die and be buried and become part of the earth that produces the grass that begins the meditations in the poem:
I bequeath myself to the dirt to grow from the grass I love,
If you want me again look for me under your boot-soles.