A difficult question to answer!
Most psychiatric illnesses do not have an identifiable cause. While we have delineated the dysfunction that occurs in mental illnesses, we are as yet unaware of the mechanisms that trigger this abnormal function. Illness can occur at any time of your life; though they are more common in the age group 15-25 yrs. There is also an increase at around 45 yrs in females. The stressful life that we lead & our family and social environment, can be contributory but are not independently strong enough reasons for causing an illness. A positive mind, an encouraging family and a supportive and caring society can help speed up the recovery and enable the patient to realize his full potential.
Current scientific thought supports the biopsychosocial model as being the mechanism that causes someone to have a psychiatric illness. This model suggests that biologically, we are all susceptible to an emotional or behavioral disorder. The start of this disorder requires a unique (to each individual) mix of psychological and / or social stress. Once triggered, the illness will then continue until treated appropriately, even if the stress is reduced or eliminated.