I don’t have a specific quotation for this point, but the encyclical opens with an insistence upon a morality that is founded on love for God and union with Christ. Without these things, Christian morality is dead. However much we value the truth itself, the Christian faith is not merely related to the brain. It must permeate one’s whole life.
The first chapter (beginning with §6) consists of a discussion of the story of the rich young ruler (Matthew 19:16-26) and its relevance for Christian morality. Christian morality isn’t about the things that we do abstracted from why we do them. It’s about more than merely doing good, or doing right. We do good because we love God; we do good because we wish to spend eternity with Him. We do not do good because we think that by our deeds we are saved; we do good because Christ says that if we wish to have eternal life—if we love Him (John 14:15)—we will obey Him. Love for God and Christian morality go hand in hand. They are inseparable. This is not to say that the man who does not love God cannot do good. We may see such an idea contradicted almost daily. But it is to say that specifically Christian morality is personal, in that it is grounded in love for God. We do not follow an arbitrary and impersonal code of conduct; we do good for love of God.
(Oh, and I would be remiss if I failed to include mention this recent brief post on the subject of the Rich Young Ruler.)