It is important to understand that Indian Camp is part of a series of stories about Nick Adams. In the context of the series, Nick is at a point where he is questioning his manhood and death. Nick is concerned he will be seen as a "coward," partly due to a comment his uncle has made. He has also begun to think intellectually about the prayer he says at night, "if I die before I wake." When he experiences the indian father's suicide, it raises concerns for Nick about death, but it also has an unreal aspect to it, as Nick is aware of the low opinion of Indians in his town's culture -- their lives not being as worthy. There is also the symbolism of the birth of the baby -- indicating the life cycle of death and birth.
Well I think mainly, its a form of denial. It's easy for Nick to think that death will never happen to him, because the deaths he's just seen doesn't remind him of himself, or anyone he knows. Basically, Nick started making the transformation to adulthood while at the Indian camp, but with his denial of his own death, the reader can see that he never fully made the transformation.