Back when I was dating a German girl - we met while we were both working at Epcot and are still good friends - in addition to experiencing a rapid and considerable improvement in my German by virtue of speaking the language with someone almost every day, I also learned a fair number of cultural tidbits you don't learn from taking German classes in college.
Like why it's VERRY important to make eye contact when toasting - or should I say Prosting - while enjoying adult beverages. The penalty for violating this particular tradition:
seven years of bad sex
In 30 years I've forgotten far more about German language and culture than I know - but I'm sure y'all can imagine why I haven't had any trouble remembering that one. :)