The Law of God serves as the rule book of the soul, the repository of values and default choices to which the conscience appeals in bringing mind and heart together for action in love.
The mind receives and processes information, while the heart stores and deploys affections. In the soul, these overlap and interact, but they require the conscience to help them process ideas and feelings into actions.
The Christian life is not all happiness and rejoicing. It is that, to be sure, but many reasons exist why Christians will sorrow and grieve, and we need to know how to do so to the glory of God.