I enjoy forming plots and developing worlds, but the main reason I write is to make my characters happy.
My characters have stories and they will stop at nothing to get them told. I wake up in the morning to them chattering in my head. During the day they pop in and out of my mind teasing me with excerpts from their lives. And, as I close my eyes at night, they tell me how scenes should play out.
They hound me day and night, refusing to be quiet. It isn't until their stories are told that they finally settle down... or, at least, step aside for the next cast of characters.