There is no one answer to this question as everyone experiences fear differently and what causes fear in one person may not cause fear in another. However, some of the most common reasons why people experience fear are because they feel threatened or are in danger. This could be due to a variety of things such as being in a dark place, being around someone who is aggressive or feeling like you are about to die. Other times, people may feel fear because they are anticipating something bad happening or have experienced something traumatic in the past.