Depression is partly a nocebo effect, in the sense....

“ Depression is partly a nocebo effect, in the sense that it can be produced by negative exceptions about oneself and the world. The way in which these negative expectations develop and produce their negative effects provides some clues as to how they can be reversed. Expectancy effects grow, feeding upon themselves. One reason this happens is that our subjective states - our feelings, our moods and sensations - are in constant flux, changing from day to day and from moment to moment. The effects of these fluctuations depend on how we interpret them, and our interpretations depend on our beliefs and expectations. When we expect to feel worse, we tend to notice random small negative changes and interpret them as evidence that we are in fact getting worse. This interpretation makes us actually feel worse, and it strengthens the belief that we are getting worse, leading to a vicious cycle in which our expectations and negative emotions feed on each other, cascading into a full-blown depressive episode. .. Positive expectancies have the opposite effect. They can set in motion a begin cycle, in which random fluctuations in mood and well being are interpreted as evidence of treatment effectiveness, thereby instilling a further sense of hope and countering the feeling of hopelessness that are so central to clinical depression. ”

Irving Kirsch

From : The Emperor's New Drugs: Exploding the Antidepressant Myth