Morrie has several rules for love and marriage in this novel. He shares them with Mitch, and shares that he believes they are the key to a successful marriage and happy life. According to Morrie, most people mess up their marriages because they don't know how to follow these rules.
His first rule is respect—in any relationship, claims Morrie, respecting each other is the fundamental thing to be successful. Compromise comes along with respect, as well, because you have to be willing to lay some things down to ensure you respect the other.
Second is holding values in common, and, in particular, holding value of the marriage in common. If you don't respect the sanctity of marriage and if you don't share the same values, your relationship won't survive.
Finally, Morrie tells Mitch, you need to be honest with one another. Without honesty, none of the other rules will matter because you can't trust each other.