What

Communication, understanding, and to fight for it

Communication, honesty and respect

For it to be meaningful. Don't go out with someone unless you feel something. And don't stay with them unhappily because of loyalty.

communication

There's no one thing there's multiple. You need to be able to understand each other, you need to be able to communicate properly, you need to trust one another, and give each check other the benefit of the doubt.

Respect, interest in ''discovering'' the other person

Trust and a good sense of humour

Communication , and transparency.

Maybe selflessness.

I think there is not a single thing to most important. A few things is important like trust, honesty, respect and loyalty.