Although the Shoshone welcomed Lewis, they were suspicious. They had recently been raided by another tribe. When Lewis asked them to travel to meet the rest of his expedition party, the Shoshone worried that Lewis might be leading them into a trap. Eventually Lewis convinced them. But when they got to the meeting place, Clark and the others had not yet arrived. So they waited.
The Shoshone were nervous. They didn't want to be ambushed. Lewis was nervous. He had to get horses or the Corps wouldn't be able to finish the expedition. If Clark and the others didn't show up soon, the Shoshone would leave and take their horses with them.