At least according to an unbelievably silly BBC story from the year 2000:
An American professor has developed a theory that Germans are bad-tempered because pronouncing German sounds puts a frown on the face.
Professor David Myers believes that the facial contortions needed to pronounce vowels modified by the umlaut may be getting the Germans down in the mouth…
Saying “u” [ü?—LH] – one of German’s most recognisable sounds – causes the mouth to turn down. But the English sounds of “e” and “ah” – expressions used in smiling and laughing – have the opposite effect.
Professor Myers told the Royal Society of Edinburgh on Thursday that frequent use of the muscles which the brain associates with sadness can adversely affect a person’s mood…
“This could be a good reason why German people have got a reputation for being humourless and grumpy,” said Professor Myers, who heads Psychology at Hope College, Michigan.
The story is illustrated with photographs of Michael Schumacher (looking wry, I’d say, rather than grumpy), Gerhard Schroeder (pensive), and Helmut Kohl (definitely grumpy).