The exact cause of depression is not fully understood, but it is likely to be a complex interaction of genetics and environmental triggers. It is certainly not due to a weak character or a lack of trying. Depression can develop following a loss, whether that be redundancy, divorce or deteriorating health. It can also occur following a traumatic childhood or life experience.

Extreme stress, such as bereavement, can trigger depression. However, not everyone who experiences trauma will suffer depression. The 'stress bucket' is a commonly drawn analogy that describes people having a bucket that can hold their stress. When the bucket brims and overflows, depression emerges. People with hard lives €“ for instance, those who have been abused €“ have smaller buckets and cannot absorb as much stress as someone who did not experience adverse situations.

From a neurochemical point of view, depression could be explained by chemical changes that occur in the brain. It is currently believed that a deficiency in serotonin (5-HT) and noradrenaline are key contributors to mood disorders. Like all theories that relate to the brain, however, the reality is likely to be far more complex.

Some prescribed medications can cause depression as a side effect, while many women claim to feel 'out of sorts' while taking combined oral contraceptives. Suffering from a chronic disease, such as anaemia, can also cause depression. Physical health problems should be ruled out before a patient is treated.