I think it's a power-thing. Some Americans follow the 'customer is always right' thinking and expect staff to be servile. But my experience in Europe is that when you interact with a waiter or a shop assistant you're interacting with an equal who happens to be serving you at that moment, and who also expects a certain something from you.

I still remember a waiter who was very disappointed in me because I didn't know which wine I wanted to have with my fish 😂 Sorry, Danes don't know much about wine. Anyway, the only time I've encountered rude staff was in Belgium and only once or twice - and probably just because we seemed impatient.