> The statistics tell a story different from those anecdotes: young Europeans are considerably more internationally traveled than young Americans, probably by something approaching a factor of ~2 in the likelihood of crossing national borders in a given year. But they're not necessarily twice as geographically well-traveled. Much of the difference is due to Europe's small countries, open borders, dense transportation network, and cheap short-haul international travel.
Means and using those means for any given particular end are entirely different things. It's hardly surprising that Americans are generally insular and uninterested in non-American things. (This is not a good thing IMO. But it's a thing.)
But more importantly, like you say, "interntional travel" in Europe includes, say, "going to the UK from France" vs the overseas 5-to-10hr+ flights that are required to cross the Atlantic to have these conversations in my anecdotes.
That's my bad for saying "international" instead of "cross-ocean" or such. So let me clarify that the jealousy here is being able to pay for flights and hotels to spend 2+ weeks across an ocean in cities with $100-or-equiv-per-night-to-$300+-per-night hotel costs. They didn't think they'd ever be able to afford a similar trip to SF or NY.
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The non-anecodotal version of this story would likely be the rise in social-acceptance and popularity of the more right-wing parties in Europe. A lot of people, younger especially, feel like they're getting left behind and aren't interested in mostly-older or born-into-more-money people's generic remarks about how it's still great overall for them.
K-shaped economies + "stay in your lane" are how you get Trumps :(