Why is free verse an appropriate form for Whitman’s poems?

It’s hard to imagine Whitman writing in anything other than free verse, a form that he largely invented. Partly, this is because of his subject matter, which is nothing less than the totality of life as it is experienced in the United States. Whitman also saw his role as poet as essentially democratic, not just in terms of subject, but in terms of method. This is his point when he says, in "One’s Self I Sing,"

One’s-Self I sing, a simple separate person, /
Yet utter the word Democratic, the word En-Masse.

This inclusion of both the personal and the collective demands a poetic form that can include a multitude of voices and accommodate Whitman’s conversational tone. Conventional poetic forms put a certain emphasis on the artifice of the poet, or the ability to do "clever" things with meter and rhyme, thus privileging, to an extent, poet as artist. Whitman’s view of poetry, on the other hand, was deeply collaborative. He believed that conventional distinctions such as those between writer and reader (or even, on a more basic level, between subject and object) could be overcome through his work. For this reason, the language of his poetry had to be close to the everyday language of the people; his rhythms had to be the rhythms of speech. In short, his poetic form had to be able to channel what he saw as the poetry inherent in all the infinite activities of life. It’s little wonder, then, that he found it necessary to invent a poetic form—free verse—that could give him the freedom to achieve those ends.