Because of the of the high volume of cars and the chaoticness of the layout of streets, cars flow more like a river than anything. If someone merges in front of you or enters the highway in such a way as to make you brake, do not take offense to this. This is not an act of an arrogant driver but merely a needed aspect of etiquette that is just accepted in order to make things move more smoothly. On the major roads, unless you are going at least 200 kph, the left lane is for passing only. This was a big difference I noticed.
People avoid that lane like the plague.
In Europe people drive at a reasonable speed or they drive at lunatic speeds. You do not want to be in the left lane for long or else you will get in the way of someone trying to break the sound barrier and get the shaming flashing of the lights . This is not limited to the German Autobahns, people drive pretty fast everywhere. Also, never use your blinkers on the major highways. Instead, kind of slink between lanes in a nonchalant way.