You are correct. But a loving relationship only exists when both people perceive that the other is looking out for the interests of both. Notice I used the word perceive. One can be putting everything into it and the other perceives that they are not and the mariage fails. And (somewhat more rare I suspect) another marriage could have a bum for a mate, but the spouse perceives he is doing good. These perceptions go right to the emotional makeup of the the person who is perceiving it. A bum or a bitch can never be pleased, because they are bitter people and they will never ever be satisfied. They will never perceive anything but their own needs and how unfair life has been to them.
I have an adopted relative like this, adopted after a harsh early life, and her mother has said that trying to fill her up with what she needs is like trying to fill a swimming pool with a colander. Nothing has seemed to be enough, and it everything has always been "about her".
I still think that there can be hope for people like this, if they will just wake up and come to their senses, and start to appreciate the good things they do have.