Germany is Europes biggest country by population and GDP (not counting Russia and Turkey for population). It makes perfect sense for them to be the biggest spenders on defence as well.
Ironically the ultra right wing AfD party is actually against the rearmament, but that is mostly because they are Putin's puppets. (They claim fiscal responsibility concerns but this is a laughably transparent excuse)
For actually qualified people and experts in the field it is a balance act between being able to push back against dangerous or stupid policies and legitimizing those stupid and/or dangerous policies by having your name associated with them. I guess the balance finally tipped for him.