For me, it was simply impatience… i was impatient for JKR to release book 5 (at the time), and impatience for a potter fanfic author to continue their story. I had a list of ideas that brewed on how that fanfic should continue, and I decided to play with those ideas, a narrative formed, and I was hooked on creative writing. I’m still working on my sequel (the million word one), one that’s so different from the above, that I’d have to rename one main character to bring it here to this archive (but alas, it felt wrong when I attempted that).
Otherwise, I prefer playing in my originals… so many things to play with.