The reality is most people are unhappy with their lives (at times) and always have been and only a small percentage are seriously unwell.
If you ask me most people being unhappy with their lives is a problem.
Still, it's certainly not that modern society is horrible. Far more people (especially in "developed" countries) live far better lives (materially, anyway) than at any time before. But maybe what's missing is meaning. Family and other social connections. Like "Why am I doing this?"
Maybe it's just an unavoidable consequence of the extreme specialization of labor in modern society. And that there are many important jobs that just aren't much fun. Or at least, aren't acknowledged or rewarded well enough. Don't come with enough status.
The UK's Mental Health Foundation gives these figures:
> 1. Mixed anxiety & depression is the most common mental disorder in Britain,
> with 7.8% of people meeting criteria for diagnosis.
> 2. 4-10% of people in England will experience depression in their lifetime.
> 3. Common mental health problems such as depression and anxiety are distributed
> according to a gradient of economic disadvantage across society. The poorer
> and more disadvantaged are disproprotionately[sic] affected by common mental health
> problems and their adverse consequences.
> 4. Mixed anxiety and depression has been estimated to cause one
> fifth of days lost from work in Britain.
> 5. One adult in six had a common mental disorder.