Yeah, I've recently been thinking the same thing, that it's really hard to explain the "why" of fanfic, because either you like making up things about other people's stories, or you don't. If you do, you'll "get" fanfic pretty easily, and if you don't, no amount of explaining will help.

For me, fanfic is more than just fic, it's a way of looking at fiction in general. Yesterday, I was talking to a couple of colleagues about The Three Billygoats Gruff, and I mentioned that I've always thought that the billygoats decided in advance what they'd tell the troll, because if they hadn't, it would have been really cruel of the smaller billygoats to sacrifice their brother like that. One of my colleagues said, "Oh, is that what you think?" (clearly interested) while the other said, "I guess I've never thought that hard about the story."

I've come to realize that some people can read or watch a story and leave it alone afterwards, even if they like it. That there will never be anything going on when the pen is put down or the camera turns the other way.

And I have absolutely no idea how to explain the "why" of fanfic to people like that.