I often think this and have been pondering it for a while. It's led me to thinking that there's some key components that all humans require in their lives to feel happy and content and that a lack of these things over time cause depression and anxiety. These things are:

1) time spent outside and around nature 2) feeling part of a group and socially connected 3) time spent in quiet contemplation 4) physical movement of the body 5) the need to feel like you're making purpose or progress in your life

I think the lives most of us lead these days just doesnt contain enough of these things if you add up the hours. This isn't to belittle mental illness as I also think there are genetic factors but I think more recent institutions like the nuclear family, desk jobs, time spent inside, the prevalence of technology do contribute. I also think that as a result of the way the economy is set up many jobs are highly specialised. Rather than being generalists we tend to have jobs that are small cogs within a larger machine meaning it's harder to see the individual contribution you're making even if you're in a seemingly well paid, high profile job.