Some people use the-coconut-versus-the-peach metaphor to describe this difference (and not just for Germans and Americans). A coconut is tough on the outside, but soft with sweet milk inside. A peach is just the opposite: soft on the outside, with a tough pit in the center. Germans are coconuts, with a hard exterior that’s tough to crack. Americans are peaches, easy to get to know, but with a hard interior. As one consultant puts it: “When peaches and coconuts meet, misunderstanding is common. Peaches can see coconuts as cold and difficult to get to know, because they don’t engage much in social conversation. On the other hand, coconuts can see peaches as too friendly, superficial and even impolite because they ask too many personal questions.”
http://www.german-way.com/how-to-tell-when-germans-are-really-being-rude-versus-just-being-german/