Back to the original question.
I play SH3 (amongst others) simply becasue it's a good
game about a topic that interests me. I disassociate the events and the politics of WW2 from the games I play that are based on that era; SH3 being one of them.
I play as Germans also in other games, for instance in COD multiplayer games I frequently play on the German side if I can. Why? Just for the fun of it.
In IL2, I frequently fly for the Luftwaffe. Why? Just for the fun of it.
In Steel Panthers, I play German campaigns. Why? Just for the fun of it.
I suppose the fact that the Germans had some technically superior equipment, or had equipment with a fearsome reputation on the battlefield, or that the Wermacht had a reputation for tactical superiority, has an influence as well.
But at the end of the day, it's just a game.