According to webmd.com, Depression is a complex disease. No one knows exactly what causes it, but it can happen for a variety of reasons.
Major Causes
Physical, sexual, or emotional abuse can make you more vulnerable to depression later in life.
Abuse
Major Causes
Sadness or grief after the death or loss of a loved one, though natural, can increase the risk of depression.
Death or a loss
Major Causes
Women are about twice as likely as men to become depressed. No one's sure why. The hormonal changes that women go through at different times of their lives may play a role.
Gender
Major Causes
Depression in someone who has the biological vulnerability to it may result from personal conflicts or disputes with family members or friends.
Conflict
Major Causes
Problems such as social isolation due to other mental illnesses or being cast out of a family or social group can contribute to the risk of developing clinical depression.
Personal problems
Major Causes
Sometimes, depression happens along with a major illness or may be triggered by another medical condition.