Yes, if the people identify as Danish you can’t just change the flag and expect them to be Americans. When I went to the Falklands it was very clear that it didn’t matter how badly Argentina wanted the islands. Even if Argentina managed to conquer the islands militarily the islanders had decided they were British and that was that.

Especially following WWII, in which Denmark offered no resistance.

Maybe that statement is a bit ambiguous and probably by a lot of views incorrect:

https://en.m.wikipedia.org/wiki/Danish_resistance_movement

Okay, fair point, we can then say little resistance.

My point was that few Danes we're proud of this, and had something to prove. Thus, selling Greenland was off the table.

Why should Danes have resisted in WW2?

Because they were occupied by Nazi Germany?

Was that bad for Danes?

Typically it is expected that sovereign states resist attempts at undermining their sovereignty.

Expected by who? The original comment I replied to said that Danes offered no resistance to Nazi occupation, as if that were to be expected without stating why.

some of them, very bad.

Maybe you're mixing that up with Poland? Nazi rule in Denmark was popular and internment camps there we're similar to US ones of Japanese, Italians and Germans, and similarly in the US there was no resistance either.