This might be controversial, and I'm not saying it has to be the case, but I believe at least part of it is due to lack of sunlight / outside movement (vs all day in office, car, train, home with stuffy air and sometimes not even openable windows) and nutrition (too much processed and fortified food, not enough protein for many people).

I'm trying to focus on both in my life, and I have become much happier as a result.