Greetings
To build and maintain, at the top; for myself, would be MY ability to trust him/her/both. And that trust has to be unshakable. If I have doubts of any kind, the relationship will suffer.
Everything after that is just extra’s that most would want in a relationship: the ability to communicate openly and honestly, the ability to laugh at themselves and each other/any given situation, the ability to be compassionate, the ability to take responsibility for oneself, etc. There are so many determining factors that it would be impossible to list them all.
On the lines of a strong healthy relationship, I would have to go with both love and respect. To me, a strong healthy relationship is one that has and can weather any obstacle; in my mind, it takes love and a strong respect for the other person to achieve this ( Please note that I said this was for ME only; my thoughts, my opinion )
My first M/s relationship was a disaster. I could blame it on the fact that I was only 18, but that would not even begin to cover the issues that were there. It broke apart because I was unsure of my place; I had doubts; this lead to the breakdown of the trust between us. Of course, my communication skills at that time were horrible L Either way, it did not last and became what many would consider a disaster.
I wish you good humor and health
jaxon