That's it, basically. They'll have to have been around each other for such a long time Germany feels comfortable around Norway. And then either Norway has to push for it, or some wacky misundertanding has to happen before Germany'd bring up a relationship.

And when they are in one, Germany might bring it up if he notices Norway's upset with him and eliminates other possibilities. In a "have I failed my responsibility as a lover in bed?" way.

Though the faster option would be to have Norway force the subject, like Italy did with his "I want you to say you like me more clearly," lines.