The main theme that is expressed in this poem is the process of becoming.
In this poem, Whitman talks about the process by which a child becomes a person. He tells us that the child goes out into the world everyday, and looks at a lot of various different things. Everything that he sees or that he is in touch with becomes part of himself. In this way, he develops his consciousness and his identity. This is the natural process of becoming, of change and of development.