Maybe it's because in the modern world we get used to everything being handed to us, to everything being easy, to always having food and drink on hand when we want it, etc. So when things don't go right, we aren't prepared for it, and we don't think it's fair because we are so used to getting what we want, and so we overreact.
Look at the things I complain about, for example. I wouldn't have the time to complain about them if I spent all my time searching for food and gathering wood for the fire, and with sheer survival to worry about I would shrug off the injustices of the world pretty swiftly. I'd get used to dealing with stressors in a positive way, and it would be good for me.
As a man, I'd also be much more satisfied in that I am fending for myself and successfully dealing with challenges more often, rather than just having everything handed to me on a silver platter, causing me to wonder what use I am to anyone in the first place.
The modern world leaves people unsatisfied and unable to deal with the real world properly.