I guess it's a form of panic in a situation where you know that really, everything is perfectly safe. A fairground ride, such as a roller-coaster, is a good example: you know that it's perfectly safe and that nothing bad is going to happen to you -- but you still have feelings of panic and you still scream. But it also gives you a thrill, and that's why you subject yourself to that kind of panic. A horror movie is similar: you know that it's just harmless pictures projected onto a wall, but it still scares you when the monster appears from nowhere and roars at you.
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