I have definitely written a happy song about someone and then we ended up splitting up, but you have to put those kind of things to the back of your mind and tell yourself that it's a good song and it works on the album.
The first principle is that you must not fool yourself and you are the easiest person to fool.
The key to this collaboration - which we undertook after much deliberation - was to stretch creatively. New characters, new locales, new form (the novella).