Depression is the "depressing" of feelings and reflects: (1) conflict about an issue, the person being torn between choices; or (2)having been unable to reach their parents emotionally when young and later tending to give up on important goals, believing they will fail again; or (3) the sense of having deep problems; or (4) a combination of these factors, which is the most common explanation. It, of course, must be distinguished from normal grief deriving from the loss of a loved one or of an ability through accident or illness.
As for the hoary "chemical imbalance" theory of depression. This derived from two factors: (1) Today's psychiatry residents receive only 10% of the psychotherapy training that they did seventy years ago and have minimal training in developmental psychopathology/ego development unless they receive later psychoanalytic training. This caused psychiatry's fervent belief that mental health treatment IS drugs, despite the pharmaceutical industry's (largely) inadequate research with benefits being overstated and side-effects down-played. Holding a fervent belief in Santa Claus and the Tooth Fairy would damage people far less.