Bad behaviour on both sides added to the confrontation.
The US government regularly broke its treaty promises - as the Sioux Chief Gall said:
If we make peace, you will not keep it
Meanwhile, some Native Americans wanted war. Early travellers on the Plains were robbed and murdered. And when some Native Americans made peace with the US government, others would stay out on the warpath - white Americans could not understand that the chiefs had no power to make their warriors obey.
In 1866, a group of Native Americans wiped out a unit of US cavalry (the Fetterman Massacre), and events like this, and the defeat at Little Bighorn (1876), made the white Americans determined to win the war.