I don't agree that this is the "one law" for relationships, but I DO think you should never make the person you're with feel lonely, especially when you are there. One of the worst feelings in my opinion.
I don't ask you to love me always like this, but I ask you to remember. Remember inside of me there will always be the person I am tonight. Scott Fitzgerald, Tender is the Night Quote Typed on Typewriter and Framed
Never Lose Yourself While Trying To Hold On To Someone Who Doesn't Care About Losing You. For those victims of Narcissists "who doesn't care about losing you" and places you on the back burner of disrespect.