Your take that depression is caused by bad home circumstances is true for some cases, but there are many causes for depression in between, most of them not even

Your take that depression is caused by bad home circumstances is true for some cases, but there are many causes for depression in between, most of them not even close to being understood. Genetics, immune disease, thyroid problems, etc. The list is pretty huge. As for expecting parents to notice; a trained psychiatric professional cannot diagnose depression by regular life observation alone. They rely on self reporting, or suicide attempts. Anything less than that is extremely hard to pick up on.