I write because I have so many random ideas floating around in my head it gets on my last nerve at times.
Fanfiction I mostly write because I can't help it. I'll seriously sit down and watch a movie, then think...
I SHOULD MAKE A CHARACTER.
It's like that ALLAH THE TIEM.
And the same kinda goes for original ideas - they just sort of appear in my noggin. Usually though everything - due to the fact that IVE A BAZILLION STORIES - swishes around for a while though.
Take HoaL. It started out as characters for some roleplay with a friend. Then I decided Annabelle - or Jazz, as she became - and Chrissie should have a story, as well as the bizarre Lugia alternate form that I'd dubbed Lucent.
In other words, I write because my brain does not shut up and stop refining.