A survival instinct is of immense evolutionary value. Nearly all creatures that have survived danger well enough to reproduce and pass forward their characteristics have done so by possessing a mechanism that causes them to be respond to danger and try to avoid death. This is typically communicated by strong emotional and physiological reaponses. Humans are advanced in this regard, in that we can conceptualize danger in our minds and experience the reactions in our body as though they were the same thing.