My mother was German and she didn't really want to teach me to speak her language. I was too young to understand that "her people" were "the bad guys" in a global conflict a couple of decades before I was born and she was probably just trying to protect me from potentially life-ruining prejudice.
So I persisted in asking questions about her mother tongue. How do you say this in German? What does that mean in English?
And one day we talked about German having a personal or informal form of you and a separate impersonal or formal you. I was a fairly young kid, maybe something like eight years old, trying to understand this alien concept my language and culture lacked and I said something like "So if I wanted to insult someone, I would use impersonal you."
Because I'm a child and my limited life experience was mostly with family and friends and I'm thinking something like it's rejecting and othering and will distance people from me and signal something like "We aren't friends and I don't like you." and they won't like that.
To my shock, she replied "No. If I'm going to insult someone, I'm going to get very personal."
So if you are "an ugly American" learning a language like French or German and you go to Europe and try your hand at speaking with locals in their language and you use the informal form of you and they look at you like "WHAT did you just say to me, bitch?" That's why.
You are being overly personal without their permission and it's offensive. It's linguistically a little like copping a feel from a chick you just met because you think she's pretty.
European cultures are much more formal and formally stratified than American culture is and I'm from the Deep South which is generally warmer and friendlier than the rest of the US. And on top of THAT, I learned German from my immediate relatives and their very close friends, chiefly the German immigrants that made up most of my mother's social circle, so I never had any reason to use formal you.
Everyone I spoke with was someone socially close to me. So I tend to default to using du rather than sie and rest assured I've gotten that look like I just did something shockingly offensive. Which is why I can describe it.
So if you are, like me, a Third Culture kid -- a product of a marriage between people of very different cultures such that you don't really belong to the culture of either of your parents -- and you know a bit of the language of one of your probably immigrant parents and can kind of follow the conversation when their friends visit or whatever, keep in mind you are growing up in a little cultural bubble -- a small enclave of speakers of that language -- and you are not really being exposed to the larger social fabric that is the birth place of that language.
So even if you take a few classes in school and become somewhat proficient in the formal written language -- something I never did because my brother took it in school and all the German moms hated the German teacher at that high school -- odds are good you are missing important social and cultural bits involved in being well spoken and articulate.
I generally describe myself as knowing conversational German. I can say "Hi! How are you?" and make small talk, but I can't really read it or write it and I can't really understand the News on TV which has too many big words I never learned.
And when a new German store opened in my hometown and I went there with my mom, anyone from Germany who spoke fluent German was, as far as I could tell, treated like a good friend of the family. So I didn't really get exposed to formal German social stuff much, if at all, growing up.
You go to actual Germany, no, those people are not your FWENDS like you are five years old just because they are shop keepers or whatever and you know a few words of the language. And then they think you are amazingly offensive and then you go on YouTube and see a zillion videos in English debating how rude the French are while the French are probably talking trash in French behind your backs about your lack of manners.
It's a cultural misunderstanding and most Americans are not making any effort to understand the cultural parts of "So exactly why are there two forms of you and what exactly is the proper use of each?"
So there's more to learning a living language -- a language still in active use in the world in various countries and cultures -- than memorizing funny-sounding words, their weird spelling rules that conflict with English and their odd grammatical constructions.
It's fine to work on grammar and vocabulary and pronunciation. But knowing that stuff doesn't really adequately prepare you for effectively interacting with native speakers on their turf in their country where they will implicitly expect people to follow certain social protocols and etiquette that your high school French teacher may have never learned if she hasn't actually lived in France for a time.
At best, it helps you bridge the gap with visitors to your country who don't know adequate English and will not care how rude you are while helping them cope in a foreign land while their hair is on fire about some problem that would be a minor issue if they spoke fluent English and were a local familiar with the bus system or whatever.