We like this Harvard Medical School article What causes depression? because it clearly lays out the various causes.

It was originally published in 2009 but updated last year.

It delves into detail on:

• mood regulation by the brain
• Stressful life events
• Medical problems (responsible for 10% – 15% of depressions)
• Genetic vulnerability
• Medication ( Be sure to read the chart of medications at the end of the lengthy article)
• As the article indicates there are a host of treatment options for depression.