What is love in a relationship? Let me first ask this question: Do you know what love is?
Love is not an feeling of the heart. Forget about anything you may have seen in books, movies or TV shows. They typically depict a romantic image of love, which is fake and superficial. Love has nothing to do with candy, flowers, cards, etc. (These can be signs or expressions of love, but they are not love itself.)
Love is a choice. It is a conscious choice, of the free will, to do what is best for another person. It is the choice to put the welfare of someone else above your own. The Christian undertanding is this: the best thing you can do for another person, is to help them receive salvation. Hence, if you love someone, you will help enable their salvation. Effectively, you become a mnister to the other person.
Now, what does this mean in the context of a relationship? This means loving the other person, by helping them receive salvation. You attend to their spiritual and physical needs.You help guide them to Christ. You use the power of the Holy Spirit, to help build their faith.
Now, this can take various forms, depending on what is the stage of your relationship. If your partner is not a Christian, this means helping them to convert. If they are Christian, this means helping to strengthen their faith.
If you are married, then this can take numerous forms. It can mean supporting your spouse, in their vocation. If you have children, then that means helping your spouse in child-rearing. It means the expression of sexuality, by which you express your physical love in the marriage bed.
So there is no one single answer. But ultimately, all these answers have the same root. If you love someone, it means that you put the other person’s welfare first.