Is the rise in the western world of 'mental illness' such as 'depression' a reflection of language usage or some more substantive reason(s)?

I take it the question you're curious about is whether we use the term 'depression' differently now than it was used, say, 50 or 75 years ago, and whether such change in the criteria for depression explains an apparent increase in diagnoses. An answer to this seems to me to depend on empirical investigation thatphilosophers are not (mostly) in a good position to undertake. It would be important to look at the studies that purport to show an increase in depression and determine whether the criteria have changed over the period of the study. Another factor affecting diagnosis is the availability of medication to treat the disorder: if there is no effective treatment, there is less motivation to get a diagnosis. The recent availability of effective anti-depressants may have had a substantial effect on the numbers diagnosed. Determining whether there has been a real change in the overall emotional/psychological states of people in the West would have to take many different possibilities into account. ...