I think what it is, is that there is this perceived notion that DND is about the PCs only and everybody else is of little consequence. Instead, DND is a communal story based activity in which people, together in harmony, tell and contribute to an ongoing story narrated by the DM. And while the stories of the PCs are important, the idea of "just" the PCs stories mattering and not the "whole story," along which it's other characters/NPCs seems strange to most players.
And some players get really defensive about it for some strange reason. Like, holy crap I ran over your dog mad defensive.