Indeed, I think you nailed it, happiness is so subjective as to be almost impossible to define, except maybe in those things that reduce it or cause stress.
For me it is the absence of anxiety, not the pursuit of more, or better or anything extra, I am base-line happy until something disturbs my equilibrium and pushes me out of that state.
Sometimes that push as a root from within and that's the subjective bit, but there are some externals that could contribute, like the social norms in the US you name for example.
Each of those , in my mind, increases the background stress level, I mean, do I want to live somewhere where everyone feels they must own a lethal weapon, and why do they feel that way, is there a pervading sense of fear ?...just as a simple example.
On a side note, I was in Copenhagen for a couple of days last week, loved it! :-)
I think one of the problems people often encounter is that they spend too much time examining and listening to what other people consider to be happiness and they may even follow those things even if it's to their detriment.
I think we're far better off by just sitting back and listening to our inner wisdom and determining what makes us happy, and then to let that stand independently of what anybody else thinks... even if you find yourself in the minority. Goodness knows, I've found myself in the minority plenty of times!
I'm glad you enjoyed Copenhagen! It's a lovely old city and there's plenty to explore there for an entire summer if one wanted to.