Easy - stories depend on failure.
If you are going to make stories by collaboration between players and GM, the players have to have a reason to let/make their characters fail. Robin Laws does something similar when he has Drama Tokens given for making concessions in Dramatic Scenes in Hillfolk (excellent looking system, BTW). Stories are built on failures, compromises and concessions - giving reasons for the players to generate those increases the odds of getting a collaborative story.
For a well regarded, general source for this, see "Story" by Robert McKee. It was written primarily as a guide to screenwriters, but, as McKee himself says, the principles apply equally to writing novels or other types of story - and they have some relevance for RPGs, too. McKee presents a simple but remarkably effective model for "making a story happen":
1) Generate a character (protagonist) with a Dramatic Need (of which more below)
2) Have the character take the most simple, direct actions possible to fulfil that need
3) Create a reason/reasons why they can't get what they need by that route
4) Have them try a different method to get what they want, instead
5) Skip back to (3) and repeat the cycle through "fail/try something new" until a story happens
You can try this at home - or give me a character with a Dramatic Need and I'll respond here; it can be good fun.
Two points to note:
- Step (3), creating a plausible reason why the character can't get what they want, is essential for story. If they get what they want, you don't have a story - just an anecdote.
- Step (4) defines what is meant by "Dramatic Need"; it has to be something the character is just not going to give up on. If they give up at the first hurdle, you again don't have a story - just a disappointed character.
What all this means for an RPG system is that, if you want to encourage generation of collaborative stories that are generated spontaneously via interaction between the players (including, but not limited to, the GM) then rewarding the players for the failure of their characters is a good idea.