Your risk for developing depression increases when there is a family history of the illness. The risk is somewhat higher for those with bipolar disorder. Not everybody with a family history develops the illness. Also, major depression can occur in people who have had no family members with the illness. This suggests that additional factors are involved in the onset of depression.
Evidence indicates that brain biochemistry is a significant factor in depressive disorders.
For example, individuals with major depressive illness typically have too little or too much of certain brain chemicals, called neurotransmitters. Sleep patterns, which are biochemically influenced, are typically different in people with mood disorders. Depression can be induced or alleviated with certain medications, and some hormones have mood-altering properties. Environmental and Other Stressors. Significant loss, a difficult relationship, financial problems, or a major change in life pattern have all been cited as contributors to depressive illness.