A somewhat unusual hypothesis is that, in modern society, we are far too comfortable and lonely. For almost our entire history as a species, we woke up every day with a very clear purpose: collect the food and resources necessary to keep you and your group (family, friends, etc) alive while also protecting you and your group from very real external threats (warfare, predation, etc). Fail to do these tasks, and you and your group may die.
Contrast that to now, an age in which people are constantly looking to find meaning and new challenges in their lives. We are not presented with fundamental life-or-death challenges regularly, and our minds are going haywire as a result.
Also, those of us living in large urban areas do not have a 'group' in the same way that early humans did. Studies show that people living in NYC are some of the loneliest people on earth, while those living in rural areas statistically have much larger networks of friends and associates and deeper relationships with those people.
The hypothesis is that, while certainly stressful on some level, these fundamental challenges and close-knit relationships gave our lives meaning in a way that modern life does not.
Again, this is just a hypothesis that I find interesting. There are many other compelling explanations.
If you are interested in exploring this further, check out Sebastian Junger's book Tribe: On Homecoming and Belonging
You cannot go wrong with Tribe by Sebastian Junger. It’s short, just under 200 pages, but it’s packed full of fascinating ideas from across the world. He talks why tribes are important and why people in modern society are hurting for more meaning in their lives. You’ll learn about different cultures, the mind and history. It’s fascinating. I could not put it down.
There's an entire book written on this theory.
I actually just got done reading it this week. It really makes you think.