The uncertainty. When you hear an emergency broadcast, often you don't know all the details right away. For example, in a flood warning, you don't know how high the water will rise or how fast it will come. It's that not knowing that scares people.
The sudden disruption of normal life. An emergency broadcast shatters the peace and routine. Imagine you're sitting at home, having a normal evening, and then suddenly you hear an announcement about a terrorist threat in your area. All your plans for the night, your sense of safety, is gone in an instant. You start thinking about all the worst - case scenarios. And the emergency broadcast might keep updating with more and more concerning information, which just piles on the fear.
The unknown. When an emergency alert comes and there's not enough information. For example, if it just says 'Alert: Danger' without specifying what kind of danger. It leaves your mind to fill in the blanks with all kinds of horrible things.
The unknown elements make a scary story terrifying. When there are things that the characters and the readers don't understand, like a strange noise in the dark or a figure that appears and disappears without explanation, it creates fear. For example, in a haunted house story, if there are unexplained cold spots or voices, it gives a sense of unease.
The idea of a wheelchair being scary is often because it's associated with the unknown. A moving wheelchair with no visible operator gives the impression of a ghostly presence.
The wendigo's appearance is very frightening. It is often described as extremely thin yet very tall with long limbs and sharp claws. Just the thought of such a grotesque creature lurking in the shadows can send shivers down your spine.
The unknown factor. In a scary online story, often we don't know where the threat is coming from. For example, in a story about a haunted website, you don't know what will happen when you click on a certain link. It could be anything, and that uncertainty is what scares us.
The Mothman's appearance. It has huge wings and glowing red eyes which are very unnatural and spooky.
The unknown elements make the Jeffy Scary Story so terrifying. We don't know what's going to happen next. For example, if Jeffy is in a strange place, we don't know what kind of creatures or dangers lurk there. It plays on our fear of the unexpected.
The grotesque creation of the monster in the Frankenstein scary story is what makes it so terrifying. The idea of stitching together body parts and animating them goes against the natural order.
The gross factor plays a big part. Boogers are, well, gross. And when you add elements like them having a life of their own, crawling or chasing people, it combines the natural disgust we have for them with fear. Also, it's often set in familiar places like a bedroom or a house, which makes it seem more real and thus more terrifying. For example, if a booger - monster appears in your own room at night, it's extremely scary because it's your personal space that's being invaded by this gross and menacing thing.
The unknown. In a mirror, you expect to see your normal reflection. But in a scary story, when something different or unexpected appears, it's terrifying. For example, a strange face or figure that shouldn't be there.