There is not only one single cause of depression!
It is known that there are biochemical changes and a biological vulnerability for depressed patients. We know that there is also a genetic influence that may cause depression: First degree relatives with depression are a risk factor for a depressive disorder.
However psycho-social factors have a big influence! According to the diathesis-stress-model of depression any kind of stressful event or unhealthy lifestile might trigger the onset of the disorder.
Possible triggers (causes for the onset) for a depressive disorder may include: