I find it interesting that there's an expectation that smart people would be happier.

Being smart is essentially "having a strong brain". Would we expect people with a strong back or legs to be happier? I wouldn't expect someone good a maths or puzzles to be any happier than someone good at carrying heavy things.

There's a difference between smart and wise. Some smart people become wise people, and I find them to be happy.

I kinda would expect a person with a strong back or legs to be happier. I think there's a strong correlation between health and fitness and happiness.