Causes of Depression:
Depression has no single cause; often, it results from a combination of things. You may have no idea why depression has struck you.
Whatever its cause, depression is not just a state of mind.
It is related to physical changes in the brain, and connected to an imbalance of a type of chemical that carries signals in your brain and nerves. These chemicals are called neurotransmitters.
Some of the more common factors involved in depression are:
Family history. Genetics play an important part in depression. It can run in families for generations.
Trauma and stress. Things like financial problems, the breakup of a relationship, or the death of a loved one can bring on depression. You can become depressed after changes in your life, like starting a new job, graduating from school, or getting married.
Pessimistic personality. People who have low self-esteem and a negative outlook are at higher risk of becoming depressed. These traits may actually be caused by low-level depression (called dysthymia).
Physical conditions. Serious medical conditions like heart disease, cancer, and HIV can contribute to depression, partly because of the physical weakness and stress they bring on. Depression can make medical conditions worse, since it weakens the immune system and can make pain harder to bear. In some cases, depression can be caused by medications used to treat medical conditions.
Other psychological disorders. Anxiety disorders, eating disorders, schizophrenia, and (especially) substance abuse often appear along with depression.
http://www.depression.com/causes_of_depression.html
