Usually, it is the soul of the future child that is responsible for the love that parents experience with each other. The soul desires to incarnate and this desire fires up the love of the parents. It is the child who makes the parents love each other as it grows up. That is the energy of the soul that energizes the love in the family.
Also, when the children grow up and don't require the support of their parents, the love between the parents often fades away. Again, you can see that it was the invisible energy of the child's soul that fueled the love. When the children move away, the parents often experience a crisis in their relationship and have to find new reasons to be together.
The soul chooses parents before conception. When the parents meet, that's when the new child's soul starts creating its future body. It fuels the love, guides them toward each other, helps them with their problems and ultimately helps them to fall in love.