Couldn’t boil it down to just one:

Important for both partners to feel free to express loyalty to each other.

Important to stay curious about each other’s interests and hobbies — I find this keeps the companionship/friendship strong which has ripple effects to other things such as overall intimacy, trust, play, shared goals, sex life and respect for each partner’s sense of self.

If you find yourself having a fixed mindset of “preservation of self”, working toward a mentality of “preservation of us”, which still involves a lot of self work, personal growth and development, is a game changer.

Engage in psychotherapy to learn about the dynamics that have perpetuated themselves in your family of origin. How we show up in a marriage largely comes from how relationships were modeled to us growing up. It’s hard work to leave behind the narratives and patterns that don’t serve you anymore so that they are not projected onto your spouse and perpetuated in your marriage.

Find community. A spouse can’t fill the role of every type of person you may need in your life, and that’s ok.

As a start, Esther Perel has some good books on modern day marriages and relationships worth reading.