A normal friend wants to immediately hear your entire history.
A German friend wants to look at you suspiciously then slowly get to know you.
Germans are not open books. They can initially come off as a little humorless and somewhat suspicious, particularly to overly gregarious Americans. My main take-away regarding the people I met during my first few months in Germany was that these people are not willing to “play along” — if they don’t like something, they’re going to let you know, bluntly and immediately.
As I got to know some of my coworkers better, my impression changed radically. After I took the time to get to know people slowly, I learned that the friendship was worth the wait.
A few liters of pils also helps this process along.