I wonder if the lack of ethnic diversity in the "happier" countries is a contributing factor:
https://www.washingtonpost.com/news/worldviews/wp/2013/05/16...
https://www.washingtonpost.com/news/worldviews/wp/2013/05/16...
EDIT: those countries are getting better, but still...
[1] https://en.wikipedia.org/wiki/Immigration_to_Switzerland
I don't think either dataset is gathered reliably, but if you're looking at a correlation there, there are glaring exceptions.
As somebody who lives in Canada, I think cultural compatibility is important, but ethnic homogeneity is unnecessary. Ethnicities are generally a proxy for cultures, Canada has a great deal of success in separating the two, though the prime minister doesn't seem to get it.
Anyway, not a hard-and-fast opinion, but I think ethnicity is a weak bond, and probably should be.