All happiness is relative to expectation.

I have lived in Thailand for the last ~8yrs. It's unfortunate not to see it on the list as I think generally speaking Thais are much happier than either US (where I travel a lot for work) or Australians (my home country).

A big chunk of that is the expectations, they don't need many material things to enjoy life and place a much higher emphasis on community and social standing (which isn't primarily derived from material wealth). Your occupation in Thailand has a very large impact on your social standing - more than the income you derive from it. i.e doctors are extremely well respected, however public doctors more so than private ones despite the latter being more wealthy.

Sense of community is something that builds you up rather than the Western trope of comparing yourself to your neighbour that breaks you down.

That very simple difference seems to have an outsized impact on how happy folk are here.

There are exceptions and Bangkok is much more Western but if you live out in the country like me then Thailand is a very happy place.

While what you said may be true, it's also likely that (I assume) being an expat you are generally living in large urban areas and interacting with the top N% of the country in terms of wealth and opportunity. Go a bit deeper and the reality may be very different.