I wonder if it's because of the kinds of stress people in western societies deal with. Humans have always had to deal with stress, but the modern world seems to have more "intangible" kinds of stress. It's one thing to be stressed about being in danger from a predator. The stress drives you to action, and you either survive or don't. But the kinds of stress people deal with now is somehow different. Most of the time, the things you're stressed about won't kill you, but the stress won't go away either. Worrying about money to pay rent, etc. The stress drives people to make enough money to make this month's rent, but that same stress will constantly be there, because of next month's rent.

I dunno if this makes any sense, just something that popped into my head.