Teaching behaviors to couples that help them demonstrate that they are not selfish is an important step. You can tell someone not to be selfish - but it helps to show them what selfish and unselfish behaviors in marriage look like. And then teaches them which behaviors are likely to help them build and maintain a good, strong marriage and which behaviors are likely to kill off the love they started out with. If I learn the skills to ski down the mountain safely and joyfully I'm going to have a very different attitude about standing at the top of a steep hopeless looking slope and very different feelings about the partner who brought me up here in the chair lift - than if I didn't have those skills. Changing my skills can change my attitudes about the challenges of marriage and about my partner and about myself.
Competence creates confidence and hope.
Helping couples treat each other with behaviors or skills that build greater empathy, dignity and understanding and with fewer behaviors that reflect contempt, blame, criticism, defensiveness should increase the amount of love in their hearts. You might think lecturing them about the need for more love in their hearts will work - I think we are safer to teach them behaviors that will lead them to changes in their feelings and attitudes. Or at the very least be sure we do both. You aren't helping couples build a partnership marriage or increasing their love and trust. That these attributes are expressed in a marriage by listening and talking in such a way as to increase the feelings of love and kindness and compassion and cooperation and success.