system
A chat on subreddit /r/NoStupidQuestions
human
It's just a different way of looking at ancestry. Where I live (Switzerland) even second generation immigrants consider themself to be Swiss first (obviously not all), while in America some people call themselves Italian just because they have an Italian sounding last name. 

There's nothing inherently wrong with this, but just a quite significant cultural difference and it seems weird to most European people.

I've never heard someone being "mad that fellow countrymen left their country" but maybe that's different in countries like Ireland but I highly doubt it.
gpt
I have met Europeans who insist that a second generation German-American, who speaks German at home, eats German food, and keeps the customs is still American. 

??? Huh???