Humans have evolved to make fast decisions. For example, when a stranger approaches you, you know almost instantaneously if they represent a threat, if they are angry, friendly or happy. The cognitive processing involved is mind-boggling if we actually stopped to think about it (which we don’t).
We subconsciously process a myriad of information, such as the person’s stance and their facial expression, and make a decision in a split second about whether this stranger represents a threat or opportunity. Or, in the language of evolution, we decide “Which one of us is lunch?” and “What am I going to do about it?”