Just like French, German has two forms of the second person singular, Du and Sie.
When I was a kid, rules were very clear. Virtually all adults were addressed formally, only relatives were Du. Likewise, almost all adults siezt (yes, there are verbs for both the forms) each other. The most extreme example is surely my father. To this day, I can count the number of people that he uses Du with on both hands.
Now, things are very confusing.
Everyone still uses Du with children. But that’s about the only clear rule that remains.
- Academics seem particularly prone to Du, possibly out of embarrassment of the implied hierarchy of a Sie, especially when it is asymmetric, i.e. student using Sie with prof, but prof using Du.
- Some salespeople will use Du for me, others Sie.
- It almost seems like there are more situations where people react unhappily to Sie than the other way around. It used to be that some people would physically recoil if addressed by Du. Now, others might be offended to be siezt. For example, one of my field hockey buddies recalled that a fellow fan at a Hertha football match used Sie with him. He almost had to smack him.
- My brother, who used to be such a rebel by simply using Du (and the first name that goes with it) with everyone, by contrast, has retreated to the occasional Sie.
- Sie is used for teachers, even by parents, and even though I would probably use Du if I met a teaching in another context (the Olympiastadium, for example).
- Obviously, hockey is all Du all the time.
It’s all very confusing even to the native speaker!