The expert consensus is that depression is the result of genes, biology, and the environment interacting with one another.

Genetics: We have yet to identify depression genes, though experts expect we will find several, each making a small contribution.

Biology: The biology largely concerns how neurons in the brain communicate to one another, and the chemical actions that take place inside the neuron, plus various stress pathways.

Environment: This part of the equation includes personal stress and traumatic experiences.

These alterations in brain chemistry which can be triggered by illness, stress, frustration, or grief, do not resolve themselves upon removal of such triggering events. Instead, the alteration continues, producing depressive symptoms and through those symptoms, enormous new stresses on the person: unhappiness, sleep disorders, lack of concentration, difficulty in doing one's job, inability to care for one's physical and emotional needs, strain on existing relationships with friends and family. These new stresses may be sufficient to act as triggers for continuing brain chemistry alteration, or they may simply prevent the resolution of the difficulties which may have triggered the initial alteration, or both.