why do people fear thigs?

Answer 1

Innate (instinctive) fear that could mean the differemce between life and death, learned fear from parents/family, acquired fear...from experience.
I guess fear was designed to keep the first humans alive. It was healthy to fear the rage of a wounded animal, certain death to face it up with nothing more than a flint spear...;

Answer 2

At the most basic level, people fear some things so that they can stay alive. People in all cultures fear dangerous animals, poisonous food and plants, and situations that put them in mortal danger.

Pain is a strong motivator to keep people afraid. The pain can be physical or emotional. Children learn not to touch the hot metal on the fireplace grate; they also learn not to go out into the snow without clothes, because both cause pain. Later on we learn (we hope) that certain relationships cause us pain, so we try to avoid destructive relationships as we move ahead.

Tyrants use both physical and emotional pain to isolate and frighten people into behaving as the tyrants want them to behave. Institutions use fear of consequences to keep people in them in line -- schools, churches, prisons, the military, social clubs, and so on. Politicians use fear to get us to support their laws and wars. And on and on it goes.