There are so many possible reasons why people develop mental illness but these are some of the most common: childhood abuse. Those who are abused at an early age may sometimes find ways to cope by becoming a sadist towards animals and other people or by retreating and just trying to make himself/herself invisible.
Loneliness is another reason for mental illness. It is true that no man is an island. People who are unable to speak about their emotions with anyone may feel more lonely than others to the point that they may become mentally ill. Another sign of mental illness is long-term stress. Those who have experienced stress for a long period of time may start to experience hallucinations just to endure the feelings of stress they are going through.