Only humans possess the advanced capacity for self-awareness. Though, in many ways, this capacity has helped to make our species the most versatile and powerful creature on earth, it also represents the source of our greatest affliction. With the advent of our species, with the emergence of self-conscious awareness, a life form became cognizant of the fact that it is going to die. All we had to do was to look around us to see that death was inevitable and inescapable. More terrifying yet, death could befall us at anytime.
Any moment can be our last.
When an animal gets too close to fire, for example, it reflexively pulls away. It is this negative stimulus, this experience we call pain, that prompts all forms of life to avoid such potential life threats. Pain, therefore, acts as nature's electric prod that incites us to avoid those things which may jeopardize our existence. Among those animals higher up on the phylogentic ladder, most particularly among the mammals, threatening circumstances elicit a particular type of pain we call anxiety. For example, a rabbit is cornered by a mountain lion.