Regarding your "circumstances" example, a hypothetical growth in bureaucracy would only be a valid explanation if the US had a relatively higher level of bureaucratization than all of those other countries that have a much lower level of mass shootings.
Hard to define 'higher level', isn't it? You can refer to public servants per capita, but would that really be telling? You can look at the amount of red tape to deal with trying to get anything done on the local level. See France for the benchmark on making getting anything small done as annoying as possible. So that could be an angle to inspect. But no, what I'm referring to specifically is the bureaucracy of un-responsibility where vast engines of government action go on with either no oversight, no transparency, or no connection whatsoever to the voters. In other words, areas of government that can't be voted on, voted out, affected by voter preference, swayed by public opinion, or moved in any discernible way. This kind of bureaucracy doesn't necessarily need to be
more cumbersome than that of European governments, just more out of reach. The sign that this exists isn't how many public servants there are, but how much the populace feels their opinion doesn't matter in what goes on and realizes there's no step they can take to affect it. And yes, America beats those other countries handily in
that type of bureaucracy. And it's not just how little affect the voters have, either, but it's the scope of the things going on that they can't effect, too. And America clearly has 'bigger stuff' going on than those other countries.
I do believe that there is some argument that the balance of power between workers and employers has been tilted in favor of employers to the greatest degree in maybe 80 years, and so that may be a force that is relatively disempowering people. But of course that is the "free market".
Tell me about it. Well, to be fair it's a 'mixed market' (combination free market and statism) but I believe the same would happen in a free market anyhow so I'm not disagreeing with you.
Regarding public mores, and the claim that " Americans have a fierce sense of taking action on problems". That deTouqueville/American exceptionalism argument might be true, it certainly is a story we grew up on a long time ago, but is there actual evidence that is a reflection of the current US population? And more so than other countries also formed by immigration such as Australia, New Zealand, or Israel?
Australians are really quite affable and not at all pugilistic in my experience, so it really is a good question why they didn't end up with that rebellious streak that Americans had (and still have). I don't know enough about Australian history to answer that, but even just the famous Australian wanderlust should, by itself, sufficiently demonstrate that how they view their relationship with their country is vastly different than how the Americans view theirs. In terms of whether the American spirit of action is still there - oh yeah, I don't think that's gone away. If anything Hollywood has emphasized it, even while at the same time increasing creature comforts stifle it to an extent. Certainly in the 20th century, all the way up to the 50's it was so obviously an American trait that there would have been no need of discussion about it. British economists noted the different work ethic among their American counterparts and how Americans seemed to almost hold in contempt the idea of rest and leisure. To what extent this has remained
as is since then is a question, but it takes many generations to change something so endemic to a people, assuming it is even in the process of being weaned out at all. You could argue about whether it's as strong a trait as it used to be, but I have no doubt that it's still a significant one.
By the way, while 'exceptionalism' is a potential takeaway from de Tocqueville when you hear him speak of these things, I personally don't refer to the American spirit to action as being a good in and of itself. I merely cite it as a potential factor in why Americans would be more dissatisfied than other peoples in feeling disempowered, and moreover, would be more prone to
do something about it. But unlike certain peoples who believe in collective action, the American spirit is more about individual action; 'what can I do' more so than 'what can we do.'