Depression has also been proven to run in families and therefore may be genetic. I am extremely curious about how this chemical imbalance results from stress. Though I was unable to find any clear-cut evidence that would explain this phenomena, I have developed my own theories about it. I feel that serious stress may cause an over-exertion of neurotransmitters. As people attempt to deal with stress they may eat, cry, physically exert themselves, or perform other activities to relieve the stress.
People also tend to think whether rationally or irrationally during depression.
All of these activities require a lot of communication in the nervous system, specifically in the brain. As neurotransmitters work double time to perform activities they may not be used to performing, they become exhausted. There are hormones that play an important role in these activities that may also over-exert themselves and people that are stressed are often more susceptible to infection, and other medical problems which cause their immune systems to work double time. The action of these systems over-exerting themselves, may result in chemical and hormonal imbalances. These imbalances disrupt the communication necessarily for normal functioning of the nervous system.