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.