Fear as a defense mechanism

Fear protects organisms against a perceived threat to their integrity or existence. Fear can be as simple as moving away from a negative stimulus, or as complex as existential anxiety in a human.

Some of the brain's main chemicals that contribute to the "fight or flight" response are also involved in other emotional states such as happiness and excitement. It makes sense that the high arousal state we experience during a scare may also be seen in a more positive light.