Expressed gratitude appears to do quiet, cumulative work inside a relationship, functioning less like a single grand gesture and more like regular maintenance, a habit of noticing and naming what a partner, friend, or family member does well, rather than letting it pass unremarked as simply expected.
Couples who express gratitude toward each other regularly tend to report higher relationship satisfaction, and the effect appears to run in both directions: the person expressing gratitude feels more connected, and the person receiving it feels genuinely more valued, each reinforcing the other over time.
The habit costs very little to build, a specific thank you rather than a general one, but its absence tends to be noticed far more than its presence, since a relationship where good behaviour goes consistently unacknowledged can slowly start to feel taken for granted by both people involved. Small as it is, that habit of noticed and named appreciation tends to do more for a relationship's long-term health than most people initially assume.