I imagine the incentive to become well enough to leave is just to be well enough... to be able to function without depending on or paying another person, therapist or not. To build a healthy attachment to them so you can internalise them and go it alone. Sort of like when a kid is happy to be away from its care givers? Though you still might want to check in with them sometimes. I may be the wrong person to talk about attachment though, I'm not very good at it.