I think a lot of it has to do with law and education. People in a bad situation hundreds or thousands of years ago were probably far more inclined to use violen

I think a lot of it has to do with law and education. People in a bad situation hundreds or thousands of years ago were probably far more inclined to use violence to improve their lives. Banditry and similar were far more common. Combine this idea with less awareness of abstract ideas and dealing mostly with concrete things probably meant that people were less worried about larger concepts such as their life as a whole and were more worried about tomorrow or today. You're probably less likely to fall into negative thought patterns that way. The other thing is that people back then weren't wealthy enough to not be constantly working. Becoming depressed and lethargic would probably mean starving in winter. So the people that did become this way probably died off for hundreds of years.