Why are hallucinations most often scary?

For some reason, the question was addressed to me, I will try to answer to the best of my ability. Criticism from more knowledgeable comrades like practicing psychiatrists, clinical psychologists, psychotherapists, neurophysiologists, and other competent individuals would be welcome.

So, before going directly to the answer itself, I would like to highlight three facts from which the answer will follow.
1) We build our own subjective (!) picture of the world based on the work of analyzers body (in addition to experience, attitudes, ideas, expectations, etc.), which are processed by the brain into a holistic picture, which can be, among other things, emotionally colored.
2) Consciousness often seeks to explain what is happening on the basis of experience and feels uncomfortable with the presence of unfinished, unexplained phenomena.
3) Hallucinations are rarely a sign of a well-functioning, rested and clear consciousness. Much more often they manifest themselves with overwork, chemical effects on the brain, with disorders of the nervous system and rather serious mental disorders.

What follows from these three facts? Let's take the simplest example, like the one glorified in the legends of the hippie era, the psychedelic revolution and the heyday of psychopharmacology. Any psychonaut knows for sure that his "journey" will depend on both his internal state and the influences of the external environment. That is, if he morally knows what he is doing, that he is safe and what awaits him, he will go on an amazing flight across the beautiful higher worlds and will communicate there with wonderful elves or touch gentle radio waves. If the same amount of a psychoactive substance is taken by a person who is worried about something, upset about something, anxious or does not know what will happen to him, or is thrown into an unfamiliar environment in this state, all his experiences will be accurately and intensively reproduced in consciousness in full growth. Thus, we observe here consciousness, placed in unusual conditions, trying to build a picture of what is happening on the basis of its distorted sensations and explain them through a certain image.

Also, for example, some patients of neurological or psychiatric departments may complain that some insects are crawling under their skin and are terribly annoying. Let's go back to the above three facts. The person experiences persistent itching. He can't find a reason for it. Somewhere out of the corner of his ear he heard about scabies mites or watched a program about African parasites and a caring mind gives him a crazy idea that this is exactly the reason for the itch that he was looking for. Normally, such a thought will go back where it came from. However, if a person is systematically overworked, has increased anxiety, drinks hard, works in hazardous industries, or God knows what else - he will say: "Yes. It is rare African parasites that crawl under my skin and I should urgently tell the doctor about it." And if he had a dormant mental disorder, then he will not go to the dock, decidein that he was infected by an infection-mother-in-law, who always hated him.

Joking as a joke, but consciousness always tries to find a reason and a complete image of what it cannot resolve for itself. And he can do this in the form of both bodily sensations and explanations of these very sensations, and voices, and even external elaborated images. And since this most often occurs against the background of the work of a weakened or sick organism, the image is likely to be, if not frightening, then at least alarming.

Hallucinations are an interesting phenomenon. Most often, you do not feel the moment of transition, and, far from always, visions are scary, it is the feeling that you see the subjective reality that scares you.

And really, why? What makes you think that most often it is scary? A completely different conversation, the fact that hallucinations are scary for a person who is not accustomed to them, regardless of the content. Throughout life, we get used to perceiving the world always in the same way, since most often we are in the same conditions. Any failures are unusual for us, it is alien to us, we do not understand what is happening to us. What we see does not fit with the usual picture of the world, sensations are out of bounds, we are allegorically transported to a foreign city without shoes and penniless, we do not know what to do in such situations, we have no conscious or subconscious attitudes towards what is happening, therefore we're just scared and waiting for it to end