I was going to respond with "Protestantism", but then I realized that I also lived in the US for ~5 years and I had no problem to adapt there. Mind you, I wouldn't go back to live in the US unless you were paying me a lot of money to do so, but at least there I felt integrated.

But honestly I don't have a better answer. Despite all my sincerest efforts, I never got the German idea of humor. I never understood their lack of flexibility in social settings. Their relationships with their family members (especially parents) is completely alien to me. I certainly don't like their "one-size-fits-all" for all sorts of socio-economic policies and how that translates to the EU. I am still annoyed by the fact that I can not find a way to go back to swimming in some structured capacity: it's either the overcrowded public pools or the "you need to be at competition-ready level to join a Schwimmverein". I find it weird that I've been living in the same building for 7 years and I know basically nothing about my neighbors - except the ones living under me who repeatedly complained about us being loud (we were not) and only stopped after taking us to court and losing the case.

German culture is very functional, rigid, and dry, and it serves people with that kind of mindset very well. However, it clashes strongly with typical Southern cultures, so it is no surprise that the parent mentioned this. People who value the "softer" aspects of life are not going to feel at home.

There are certainly exceptions, and YMMV.

I'm also a foreigner in Berlin, and my guess is twofold:

1. Their German isn't good enough, which limits the pool of people you can connect to

2. They only or mostly hang out in the expat-scene, which limits the pool of people you can connect to