What characteristic of a mongoose makes it hard to frighten one and accounts for the mongoose motto: "Run and find out"?

The motto stated in the question comes from Rudyard Kipling's short story "Rikki-Tikki-Tavi." The motto perfectly embodies the central character, Rikki-Tikki. "Run and find out" refers to the natural curiosity of a mongoose. The story says it is nearly impossible to frighten a mongoose because they are so consumed with curiosity. That means when a mongoose comes across something that should be terrifying, the mongoose isn't scared, just more. This holds true in Kipling's story. When Nag and Rikki-Tikki first meet, Nag raises himself up and spreads his hood open. The text says Rikki-Tikki was momentarily frightened, but then remembers he used to eat snakes. From that moment his curiosity takes over, and Rikki-tikki begins questioning Nag, not fearing him.