This is absolutely number one in my list of keys. An adult relationship requires two adults. One point five or one point nine is not enough.
David Schnarch, author of several excellent books on sexual and marital therapy, wrote that a healthy relationship requires partners who want to be wanted more than they need to be needed. Stevie Wonder notwithstanding, real security is not “someone who needs me,” but someone who is just fine on their own but chooses to be with me.
One point I’d add to the list is good manners — showing one another the same degree of courtesy and respect you’d show a stranger in a public place. This is part of how you create safety in the space you share.