It would be weird for happiness to increase with prosperity. Think how much more stuff we have than people in the stone age. We should all be delirious with joy, all the time, if we expected happiness to increase with material wealth. Any description of happiness that makes sense would have to say that it's some sort of local comparison, across various axes: how am I doing compared to previously/expectations/other pe…
In 100 years, people will look back and say things like, "How did they survive 100 years ago? Some countries didn't even have decent governments, let alone proper healthcare for everyone. Some people lived on the streets. People could just walk up to you and kill you! Bang! You're dead.
Now that we've got personal electron shields and everyone is guaranteed food and medical care, people should be a lot happier than back then."
But in the end, it's all relative. We look at what we have and what we didn't have, we're thankful for it, and we go on. People in third-world countries are happy, too, despite not having all the modern conveniences.
And there are plenty of miserable people because people are people, in the end.