You are hiking in the Serengeti. A twig snaps in bushes a few feet away from you. Is it the wind, or a lion?
As someone whose ancestors jumped to a worst-case conclusion (and as a result were less likely to become someone else’s lunch), we humans instinctively react with alarm in an uncertain situation. Fear of uncertainty is rarely warranted in the modern world, where stress hormones are more likely to shorten your life than to extend it.