at least according to American media, which routinely reinforces stereotypes like the English having bad teeth, Germans being frightening, and American Chinese restaurants serving dog.
At least I understand why someone would think the last one is true, but what's up with the other two?
I read a little bit on why, and if I had time, I could find that article and link it. It might be on Snopes.
The important thing to note is that crooked teeth are not a sign of bad oral hygiene. Maybe it's because Americans have this obsession with beauty, and we throw all our kids in braces (who can afford it) at the first sign of crookedness. And certainly I recall some pictures of Brits with wonky teeth. The article points out that Brits actually have one of the best oral hygiene rates, so the joke's on us.
I'm not sure if German just sounds angry normally or if it's an association with Hitler's speeches. Aside from hanging out with actual Germans, I think Americans get their exposure to Hitler speeches, which are certainly passionate. I'm sure most of his speeches were not about "Kill everyone you find and murder their children." I bet a lot of them were the same kind of speeches other leaders give to boost morale. But there is forever the dark cloud of the Holocaust looming over those speeches, so even though I bet 0% of his speeches involved, "Fire up the ovens," history has forever overlaid that sentiment on his speeches.
Now I've heard German being spoken in nice conversant tones, and it's not frightening, but nobody keeps track of nonthreatening speech. Hitler, however, stands out and is a continual black eye in the realm of stereotypes. There does seem to be a bit more throatiness when I hear German versus Spanish, Japanese, or French.