The best way to explain this principle is to think of people in love. Some people you are attracted to is not what I'm thinking of. There's something about their face, their style, something that makes you like them. That's not loving. That's infatuation. That's physical attraction.
How can you tell if you are in love? You can tell by "the law of the gift." Are you willing to give up everything for the one you love? You think so?
Will you give up your beard?
Will you give up football?
Will you give up smoking?
Will you give up alcohol?
Will you change careers?
And this is done willingly! It is not a command. You WANT to give up something very important to you because it means nothing compared to your love for the other. That is your GIFT to your beloved. And the gift of giving makes you happier than you ever imagined. That's the Law of the Gift.
St. John Paul II formulated this principle, "the law of the gift."