Personality
A University of Washington study conducted by the late psychology professor Neil Jacobsen revealed that personality type may play a major role in major depressive disorder. 78 people diagnosed with depression at least two years prior were studied and treated with cognitive behavioral therapy. During that therapy, their personality types were inventoried, and 44 percent of those patients had relapsed into depression. The people who were more likely to relapse were those who had aggressive personality types or had little dependence on other people. Researchers postulated that both personality types would leave little room for a social support system.
Cultural Influence
Cultural influences and expectations could cause a major depressive disorder, according to the book "Ethnicity and Family Therapy" as edited by Monica McGoldrick, et al. Some cultures outside of America emphasize the man as the main breadwinner. If a family immigrated to America and the father was unable to find a line of work in his field while the mother was able to make money with her hobbies and skills, it wouldn't be uncommon to find depression in the father. The inability to adapt to a host country lifestyle can also trigger depression.
Substance Abuse Withdrawal
Substance abuse withdrawal can trigger the onset of major depressive disorder. Abuse of amphetamines or cocaine can flood the brain with pleasurable neurotransmitters like serotonin or dopamine. When these feelings end, abusers tend to search for the next high to keep those levels up. During withdrawal, when those pleasure neurotransmitters are lower than average, it isn't uncommon for a patient to have feelings of depression or have a major depressive episode.
Learned Helplessness
According to behavioral theory, humans can be driven by rewards given as a result of their actions. Major depressive disorder can be viewed in this light. Some patients, consciously or not, can become depressed if it gives them a rewarding feeling, such as attention from family and friends. This can be especially true for patients who wouldn't otherwise receive it.
Medical Conditions
Sometimes other medical conditions can lend themselves to depression. Cancer in particular can cause depression. A major depressive disorder can be brought if the condition is terminal and the patient not ready to die. Body image, continuous medical treatment and paying for those treatments can all lead to depression.