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There are good replies here already, but I just want to emphasize the role of bragging. It seems boasting about yourself is quite accepted and sometimes perhaps even expected in the US. In Europe it is not at all, and we tend to react strongly to it.
Whenever I’ve found Americans to be insufferable they’ve always been bragging or taking themselves too seriously.
Not all Americans obviously. And I guess a lot of Americans can’t stand these people either. But it’s still a common American trait that very few Europeans will have patience for. Even our narcissists have learned to pretend to be humble.
There are several mammoths (instances) peacefully grazing on the same pastures (the social web), but the pastures are also shared with completely different species of animal. Perhaps they are single-user instances as no birds are landing on them; perhaps they are completely different pieces of the social web, such as Peertube or Lemmy.
Or maybe I’m over-thinking it.