The passage is: "I celebrate myself, and sing myself, / And what I assume you shall assume, / For every atom belonging to me as good belongs to you". So, the word that best summarizes Whitman's perspective is "belongs". 'Song of Myself' is a poem in which the poets shows himself naked in front of nature to symbolize the union with everything that is around: America and every single leave of grass that grows in the soil, as well as every particle in universe. The famous catalogues in the poem are efective ways of searching and finding the place where the poet, and the reader as well, belongs.
Also, the first sections of the poem depict the encounter of two realities: individual and colective. As a democrat, Whitman uses the metaphor of grass to reconcile both elements, to show people that they belong to each others. That's why in later sections of the poem, Whitman compares de democratic way with erotic relationships: they have the same passion, the same feeling of belongment.