John 3:16 states that God so loved the world that He gave His only begotten Son, that whosoever believes in Him should not perish, but have everlasting life.
God loved the world first. This love made Him to give His only Son to die for us. This can also be applied in relationships. Paul tells husbands to love their wives, just as Christ loved the church.
When one unconditionally loves his or her spouse (i.e. loving them with no reservations; forgetting past wrongs, misunderstandings, etc.), this ignites a new or renewed passion in him or her. This new passion can make a person do things for his or her spouse which would in turn revive or even increase or create strong love in the marriage.
Man had completely forgotten about God, after his fall. But God took the initiative to love us, and based on that love, gave His Son to us. This really set the stage for man to love God back. After showing such love to man, some men also showed love back to God. These were the early Christians, comprising of the disciples, some Jews and gentiles. Many of these people laid down their lives and made many sacrifices for the work of God. In marriage or any relationship, when there is so much bitterness and one spouse decides to lay everything aside and starts showing love, love would once again be ignited in the relationship. Love produces love and passion, just as hate produces hate and bitterness.
The love which God ignited in man by sending His Son to die for us still operates today. We should take a cue from this and start igniting love in others by showing them love.