This is not specific to Yellowstone, but I really like it:
Stephen T. Mather, NPS Director, 1917-1929:
"The parks do not belong to one state or to one section.... The Yosemite, the Yellowstone, the Grand Canyon are national properties in which every citizen has a vested interest; they belong as much to the man of Massachusetts, of Michigan, of Florida, as they do to the people of California, of Wyoming, and of Arizona."
And this:
Edwin C. Bearss, NPS Chief Historian, 1981-1994:
"As we Americans celebrate our diversity, so we must affirm our unity if we are to remain the 'one nation' to which we pledge allegiance. Such great national symbols and meccas as the Liberty Bell, the battlefields on which our independence was won and our union preserved, the Lincoln Memorial, the Statue of Liberty, the Grand Canyon, Yellowstone, Yosemite, and numerous other treasures of our national park system belong to all of us, both legally and spiritually. These tangible evidences of our cultural and natural heritage help make us all Americans."
John Muir:
"No wonder that so many fine myths have originated in springs; that so many fountains were held sacred in the youth of the world, and had miraculous virtues ascribed to them. Even in these cold, doubting, questioning scientific times many of the Yellowstone fountains seem able to work miracles." OUR NP, pg.54
John Muir:
"A thousand Yellowstone wonders are calling, Look up and down and round about you!'" OUR NP, pg.74