It always amazes me how the societies in the west the Roman conquered, (Gaul, Iberia, Britannia) were always militarized, no matter what. they were always able to drum up armies and decent equipment, and they were passively raiding each other. Yet in the Americas and Africa and elsewhere, similar 'primitive' societies, while not always pacifist, are generally more peaceful. When the Romans moved into Celtic land, for example,it was considered a given they were hostile to each other. Peace never had a chance. When the English and Spanish moved into the Americas, they were often honored as guests and welcomed, and the societies there were not usually based on violence and raiding. (not always, of course) Why is this?
I think why this would happen is because Celts and Romans were in (around) the same place as each other and Romans already kind of knew the Celts since the beginning (I don't think I have to source this, just take Battle of the Allia for example.). But the Europeans and Indians did not. Because the Indians never actually knew the Europeans the Indians saw the Europeans with some amount of curiosity yet still some distrust. Yet I do not know about any Indian traditions that has to do with outsiders or something. Hopefully somebody would reply with some information like that. But, I do not think Indians are as peaceful as media depicts them, I mean, the tribes were at war a lot but that is also with the Celts. But that's besides the point because we're talking about the Indians with Europeans not Indians with Indians.
So, in my opinion it's because the Celts and Romans knew each other but it's not the same with the Indians and Europeans.
Though that's just my personal theory, hopefully somebody in the replies can add some information.