I'm not German, but don't the Dutch also complain a lot? They are even more honest than Germans in my experience. But the Netherlands are much happier in this index, so I am not sure this argument holds.
As a German living in NL: Same level of complaining, just much more attention to socialisation in general. It's more of a we mentality, in Germany you can live next to your neighbors for 10 years without ever having talked to them and the workplace can be a completely impersonal space. In NL everybody knows each other, private matters are shared freely even in the work environment, transparency, group responsibility and team building are highly valued.
Socialization is highly correlated with happiness. So maybe complaining is orthogonal here.
I am Dutch, and I agree the Dutch complain a lot. For example, when travelling, and looking for the right gate at the airport for the flight back home, it’s often easiest to look for the area where people look like they’re discussing the stern letter they’ll be sending the hotel management when they get home. Generally though, I think the Dutch are pretty happy, although I’m unsure how recent political developments will impact that.