No one's quite sure of the exact cause of mental illness, but most current theories suggest that it has something to do with chemistry of the brain. There are a bunch of different things that have been identified as having a possible role in causing or triggering a mental illness to develop.

Genetic factors, such as having a parent or close relative with a serious mental illness (like schizophrenia or bipolar disorder) will mean that a person will have a slightly greater chance of developing the condition.

Stress also does seem to play a part, and although we don’t really know if it can cause mental illness, we do know that it can act as a trigger for the illness or make it worse.