Halloween -- the one holiday that celebrates, and perhaps even thrives on fear.

So what is fear? And why do we respond differently to being scared?

"It initially came into being to cue us of survival threats," said Dr. Shmuel Lissek, of the University of Minnesota Department of Physchology. "It's what motivates us to study for tests, the night before the test. It's what motivates us to be careful when crossing streets."

Explaining why the experience of being scared is enjoyable to some but not to others can stem from one's biological factors.

"Some of us just don't enjoy that process," said Dr. Jessica Siegel, of the University of St. Thomas' neuroscience program. "Perhaps at a biological level experience that to a greater degree so that becomes somewhat more aversive."