A certain percentage of more severe cases are intrinsic and unrelated to exercise, lifestyle or upbringing. And others involve such a dearth of energy, and often “leaden limbs” syndrome, such that getting out of a chair is an ordeal, that hearty exercise is not at all realistic. I fully agree that everyone who can exercise should, but not everyone can, and the implication that depression would disappear if everyone did is false.
So are you saying that an ethic or habit of regular consistent exercise would not help the majority of people to fend off or mitigate severe depression?
Being healthy makes people want to exercise. Therefore, people who are healthy exercise.