Reinforcement Contingency Theory
Reinforcement contingency theory posits the belief that one cause of depression is the loss of reward contingencies that are adequate for the individual's personal need. These rewards could be a satisfying job or the ability to engage in personally rewarding activities such as hobbies, sports, etc.
When these reward contingencies are lost, either through the loss of a job or the inability to engage in previously rewarding activities depression can be a result. As we know from classical conditioning, the loss of rewards for positive behavior leads to eventual extinction of behaviors. The extinction of a large repertoire of behaviors can lead to lack of responsiveness and and emotional/intellectual arousal and thus lead into depression.
In humans, this frequently occurs when a person reaches retirement age and finds themselves at home without the stimulation of work. When a person retires, and has no hobbies or other interests to fall back on, they can frequently suffer from this type of depression.