Why do peoples' jobs make them so miserable?

I ask this because I know I'm not the only one going through this. And you know, excluding the obvious reasons, if there is some specific problem at work, that's understandable. But what if there are no significant problems at work that you can think of, its relatively reliable and consistent, if a bit repetitive, its not incredibly stressful, yet you're still so drained and miserable at the end of the day? I just wonder if there's some common factors, maybe less obvious or even subconscious, that cause this. I just wonder, because I know its not just me, but a lot of people I know that go through this and can't pinpoint exactly why.