"Nowadays they reckon stress and depression is mental illness. I always have had a job that involved that mildly and occasionally"

I think we all have jobs like that. But clinical depression is not like that occasional 'down'.   My father in law was brilliant and had a senior position in the World Bank, but suffered greatly from mental health issues. Each of his children has clinical depression, and all but one of their children also have severe mental health problems. We are talking of people being so ill they attempt suicide multiple times, or have to spend weeks in mental hospitals in such deep despair that they spend all day every day crying in anguish, not just tearful.