There is no one cause of depression. There seems to be a familial aspect to depression but not everyone who has a family history of depression gets it, and those that do may not have a family history. What is known is that depression is associated with changes in brain function or in brain structure.

People who have had major changes in their lives may also develop depression. A medical change such as major surgery, a heart attack or a disease such as cancer or hormonal conditions may bring on depression. A sudden change in life, loss of a loved one, end of a relationship, being laid off from a job or any other stressful event may also contribute to a depressive episode (National Institute of Mental Health).

People who have depression -- or, in more formal clinical terms, major depressive disorder -- experience at least five of the following symptoms, which must include the first or second, nearly every day, all day, for at least two weeks:

• Persistent depressed mood, including feelings of sadness or emptiness
• Loss of interest or pleasure in activities or hobbies that were once enjoyed, including sex
• Feelings of hopelessness and pessimism
• Feelings of guilt, worthlessness, and helplessness
• Insomnia, early-morning awakening, or oversleeping
• Loss of appetite accompanied by weight loss or overeating accompanied by weight gain
• Decreased energy, fatigue, and feeling "slowed down"
• Restlessness and irritability
• Difficulty concentrating, remembering, and making decisions
• Thoughts of suicide or death (not just fear of dying) or suicide attempts
• Persistent physical symptoms, such as headaches, digestive disorders, or chronic pain, that do not respond to medical treatment and for which no physical cause can be found

(National Alliance for Research on Schizophrenia and Depression)