According to a paper published in the journal, Social Psychological and Personality Science, couples who express gratitude to each other, tend to invest more in the partner's sexual pleasure, leading to greater mutual satisfaction.
Specifically the study, by Ashly Brady et al, looked at whether gratitude improving something called sexual communal score (SCS) and indeed it did.
It seems to be intuitive that couples who express gratitude, would be more aware of the other person's needs and there will be more satisfaction overall.
If you are intrigued, and would like to start with your own effort on the basis of these findings, a gratitude journal, along with a reminder to write something in it, and another reminder to tell your partner what you have written may be a good idea. It is likely going to be worthwhile on its own, but if you need more incentive, sexual satisfaction is as big as it gets!
And in case you just want to measure and index the intimacy in your relationship, you can do so in this very quick self-assessment!The Intimacy Self-assessment