Depression is not imaginary. Well, not always. It’s not always simply physical either. You ask google what causes depression and you get all sorts of answers, but they mostly seem to indicate there is an answer.
Society’s Limited View on Depression
Society has an extremely limited view of the human person. Non-theists hearing that will shake their heads waiting to hear something about spiritual components of persons (they’d be right to expect it but still fall woefully short of the full picture); Christians hearing this will offer a resounding “no duh”.
But in this society, you’ll find non-theists denying any aspect to a human being that is non-physical. The most important problem facing the hungry and homeless is that they are hungry and homeless, full stop, look no further beyond that present state.