It's probably because you like the person who's making it for you and therefore it tastes good. You're appreciating the food they made for you because you didn't have to make it yourself. And then there's this the person making it just their good rubs off into the meal they're preparing for you so that makes it taste good. Really though I've got no idea about that just I know what you mean though....
Just wondering if it has anything to do with a sense of smell. My father had a cafe and after smelling food all day I wasn't very hungry. I've heard people who work in bakeries say that after awhile they don't want to eat the pastries. So, maybe because you are smelling the food cooking, you don't have an appetite. But if you smell the food cooked by someone else, you haven't been smelling it for a long time and then you are hungry. Food always tastes better when you are hungry.
Happens to me too. I cook a lot but sometimes I wish others would for me as I enjoy it better.
I guess it's because we were around the food the entire time it was getting prepared. We smelled every compnent that went in it. So by the time it's cooked, it's a little bland and boring and we're a little less excited than we hoped for.