What's wrong with the population in Berlin?

The majority of Berliners - and Germans - don't think involvement at the level being asked is appropriate. The soldiers are reflecting the citizens' wishes. Germany's stance, as reported by the media, is that aiding Ukraine in the ways being discussed could lead to broader war and additional conflicts. Given the nation we're talking about, I'd say they know.

Germany sits along Russia's border; we have no expertise over them. Not every nation on the planet is quite as impetuous or bloodthirsty as the US.

I'd assume a lot of Germans believe the US and UK blew up Nordstream and essentially bankrupted huge swathes of the German middle class. I'm not surprised that seeing their military hardware sent to Ukraine is making them wonder if they are being dragged into a war they never chose

Proxy war.