I would say it was a physiological reaction to the unknown. It doesn't require a monster to cause fear... or bodily harm. It could be that you have to give a speech. Maybe you are going on a first date. Whatever the case, you take someone from their comfortable routine and toss a monkey wrench into it.
I've never been attacked by a bear... It just doesn't happen to me, so I guess I'd be pretty freakin' scared if Baloo decided to gnaw on my noggin' as I walked out to my Jeep today. So would most people. Maybe it would be worse for someone who had been attacked by a bear. They know what's coming... I still think the first attack would be the most frightening. The second time you might know better what to do to get out of it...if you can get out of it.
So this line of thought begs the question. How do you capture that for a reader... What do I write to scare people.
I like to take normal things... a child, a walk home, or a neighbor and turn them into something unexpected. Base the story in reality so people can relate, and then twist it just a little. I hope that's what makes people pick their feet up off the floor and check the closet before going to bed.
Having tense situations blow up in unexpected ways is another good tactic. A simple delivery for instance. Pizza guys/girls never know who's gonna answer the door or what might be going on in that strange house. What if something was laying in wait? It's tough to know what gets under people's skin. And what's terrifying to one may be laughable to another. I wish I could poke around in other people's brains (figuratively) and see which buttons needed pushed the most often. Maybe one day I'll figure it out.