> America has undergone a VERY rapid secularization.

I'm not so sure of that. America has rapidly moved away from believing in some kind of magical spirit in the sky, but they most certainly haven't given up on religion in general. They have latched on to other blind faiths and rituals.

What hasn't typically come with those new religions, like you allude to, is a church; a place where fellowship occurs. That is a reasonable possibility for the decline in happiness. Research regularly suggests that most people find happiness in relationships with other people.

Nothing is ever single-faceted, though.