The causes of depression are complicated. In general, they are related to genetic, physical, psychological, and environmental factors. If there is a history of depression among your family members, you are probably more susceptible to it. Since different people react to life events differently, their susceptibility to depression varies. However, there are cases of clinical depression where there is no apparent cause.

No matter what the causes are, in most cases, sufferers recover eventually.

To make it easy to understand, we may see it from two angles, the "Brain" and the "Heart".

Cortisol	When a person is under stress, the brain will stimulate the body to secrete a stress-related neurochemical called cortisol. Cortisol is essential to keep us energetic. However, if the level of cortisol is too high, insomnia and loss of concentration may result. Depressive symptoms may also emerge.
Serotonin	As cortisol level rises, neuro-messages to secrete another happiness-related neurochemical called serotonin will be blocked. Prolonged high level of cortisol will damage body tissues and cells, and it will take a long time to repair and recover. Many patients with depression suffer from malfunctioning of serotonin secretion. In combination of poor sleep and weakened immune system, a vicious cycle of depression will be formed.
Under this mechanism, antidepressants are used to regulate neurochemical levels in the body and thus help to relieve sufferers?depressive moods.