A fixed mindset applied to a relationship assumes that once things improve, they should simply stay improved without ongoing effort — as if a "good" relationship shouldn't require continued maintenance. This assumption is a specific, common trap: real improvement gained through a deliberate strategy (like consciously noticing and rewarding a partner's positive actions) tends to erode if the strategy itself gets dropped once things start going well, precisely because the improvement was never automatic in the first place — it was the direct product of the effort.
The growth-mindset alternative treats maintaining a good relationship pattern the same way it treats developing any other skill: as an ongoing practice, not a one-time fix that, once achieved, no longer needs attention.
Dweck describes a father, Wes, exhausted by nightly conflict with his uncooperative son Mickey, who tried praising the boy's efforts and good behavior instead of only punishing him — and it worked immediately. But once Mickey's behavior improved, Wes reverted to his old fixed-mindset assumption that the change should simply hold on its own, stopped using the strategy, and the conflict returned worse than before. Dweck describes a parallel case with a couple nicknamed 'the Bickersons,' who made real progress in counseling by consciously noticing and rewarding each other's thoughtful actions — and then, once things improved, assumed a healthy relationship shouldn't require continued effort, stopped the practice, and relapsed into worse bickering than before.