I admit I'm annoyed by adults who don't finish their plate in self serve situations (like some office cafeterias). If they didn't want that much food, why did t

I admit I'm annoyed by adults who don't finish their plate in self serve situations (like some office cafeterias). If they didn't want that much food, why did they take it? I guess it's not that much food waste, but it seems like a tragedy of the commons waiting to happen when the food is marginally free.