God isn't forced to give grace, because there's no necessity in God, but when he grants grace it finds an openness in our human nature. We have been created with this orientation toward God, but grace is entirely by the free will of God. As soon as God created us, he implanted within us an orientation toward himself. Grace, hence, is fitting to our nature. It's not exterior.
Grace heals the wounds of our nature.
Chesterton, however, stressed the importance of the supernatural for the natural, saying that if we take away the supernatural, we don't have the natural but the unnatural. The gift of grace makes us more human, more sympathetic, more fully integrated. The supernatural fits nature and the saints moved by grace are more natural. This is because grace corresponds to the structure of our being. Our nature doesn't have the capacity to grab God.