It's kind of an interesting question. What makes us inherently unhappy?
I think if the theory goes that from a evolutionary standpoint we psychologically are still better equipped to be hunter gatherers, we should assume that our feelings towards homicide and child mortality are comparable. So how happy can a people be, when 40% of their children die and another 20% die by homicide?
If we follow that thread I would argue that it's very unlikely that people were happier back when or would be happier today, unless some other component of being hunter gatherers makes us fantastically ecstatic.
What makes us unhappy are the things that the modern world takes away from us. Sense of agency, sense of community, belonging, autonomy, recognition, and many other factors. The modern day human brain and mind is still lagging far behind our current predicament. We evolved to thrive in small village cohorts that condition for small social interactions that have real impact on our lives. Here's a striking example I remember. https://www.youtube.com/watch?v=KFOhAd3THW4 There are better longer videos of the citation from the mothers side, where she talks about how alien and cold modern day society is compared to her humble village life. No amount of medicine, material possessions or modern day creature comforts could keep her in New York. she chose to leave and come back home because that's what made her happy.