Well take their observation: all things have life. They believe life is spirit. That is just their definition of spirit. And that life is a powerful one. Some natives had the sun and moon as gods, not a supernatural being. They recognize the way of the seasons, the lunar cycles, the stars, etc... Their observation took them on vision quests, and took them closer to nature than many who believe in blind faith. Perhaps what they believe comes from observation, and not a book.
It's funny when you talk to native's about spirituality compared to christians. I once asked a choctaw why he believed what he believed, and he said... "One morning I was working on a new spear, and a wind blew through the trees. It blew so quietly but all the birds quickly flew into the air. What was it that the wind told them? And then it blew through me, not around me... THROUGH ME!!! It was then that I knew. When you listen, the earth can speak to you. I believe in what I see, what I hear, what I smell."
Now when you ask a christian that, they would say... "Because god said so in his word."