Thank you! So far, I think it’s several things:

Shared worldview and goals.

Respecting each other.

In disagreements, remembering who the other person is and that you’re on the same side.

Intentionally speaking up about both good things and problems in the relationship; not being afraid to show unreserved affection, and not being afraid to say something was hurtful or caused a logistical problem and talk out a solution. Basically, clear communication and active listening.