Love of God and love of neighbor fulfills the Law. Love of God and love of neighbor is the foundation for morality. We cannot grow in our relationship with Him if we do not love our neighbors. We are to hate that which is evil.
If we do not love other people as we love ourselves, then we do not really understand what love is. If we do not correctly grasp the concept of love, then we cannot have fellowship with God because He is love. Those who allow hatred to rule over their lives are acting carnally. Hatred is destructive to who we are as people.