Writing fanfiction is a lot easier than writing original fiction.
With fanfiction, you’re borrowing characters and places from other people. You know who these characters are; you know when they’re acting out of character (at least, you think you know when they ARE in character). And in my opinion, it’s a lot easier to get inside their heads.
Original fiction is different. This time, it’s all you. They’re your creations; you are responsible for them and for what happens to them.
But in both cases, you get to immerse yourself in some pretty amazing worlds.