Causes of Depression (Etiology)

My own view is that depression arises out of an enormously complicated, constantly shifting, elusive concatenation of circumstance, temperament, and biochemistry.

–David Karp, An Unwelcome Career 

I shall never learn what “caused” my depression, as no one will ever learn about their own. To be able to do so will likely forever prove to be an impossibility, so complex are the intermingled factors of abnormal chemistry, behavior and genetics. Plainly, multiple components are involved—perhaps three of four, most probably more, in fathomless permutations.

–William Styron, Darkness Visible

Some causes (etiology) of depression

• Stopping smoking or alcohol suddenly (William Styron, Les Murray)
• Withdrawal from prescription or illicit drugs
• Traumatic event or loss
• Disease, such as cancer
• Brain injury
• Incomplete grief (ex: early loss of a mother)

There is no single consistent cause, but a combination of genes and the environment. There is usually a predisposition to depression (genes - family history of mental illness), and stressful events (environment), such as loss of a loved one, can trigger a depressive episode. 

Primary paradigm of contemporary mainstream Western psychiatry is said to be the biopsychosocial (BPS) model, incorporating biological, psychological and social factors.