Why Fan-fiction?

Today I was talking to one of my friends, he's highly addicted to fan-fiction, most anime fan-fiction though.
I asked him why he didn't write any fan-fiction himself, and he answered that he couldn't write fan-fiction and he claimed that it was the hardest type of story to write.
I told him that fan-fiction is the easiest thing you can write, he then said that Fantasy was the easiest thing to write, because then you could write whatever you want.
Which I don't agree with at all, I have read a lot of Fantasy and you have some strict lines there too, but the hardest thing with Fantasy is to write something that haven't been written about before, and you have to have some very clear images in your mind.

I like writing original fiction myself, but I have to be inspired by something. I like to make up my own characters, but I always need someone to base them on, then it's easier to see what that person would do in different situations.

So, I wonder now, what drives you to write fan-fiction?
Beside the obvious reasons, to get readers, because at this site people will just read fan-fiction.