Bishop Robert Barron in a very effective way explains the necessity of joy for all of us who profess to be Christians through the eyes of Saint Thomas Aquinas,
"In the opening chapter of the Gospel of John, we hear about two young men who, at the prompting of the Lord, come and stay with Jesus. So thrilled are they by this encounter that they immediately begin to announce to anyone who would listen that they had “found the Messiah.” In that little episode, we see the fundamental rhythm of effective evangelization: they meet Jesus, they find the experience life-enhancing, they want to tell everyone about it. The very best bearers of the Gospel are those whose joy in Christ is contagious.
The second part of Thomas Aquinas’s masterpiece the Summa Theologiae deals with ethics, the question of how precisely we ought to live. It is most instructive to note that this massive treatment of Christian morality begins with joy, what Thomas called beatitudo. Ethics is all about what makes us happy. After determining that wealth, pleasure, power, and honor, though good, are not the source of true joy, Thomas argues that only the infinite good of God satisfies the deepest longing of the human heart. Next, Aquinas analyzes the habits and virtues that inculcate in us the moves that properly order us to our ultimate good. And finally, in question number ninety (!), Thomas broaches for the first time the issue of the law—and thereupon hangs a tale. Laws, he argues, are those prescriptions and prohibitions that place in us the habits that produce the virtues that in turn give rise to joy. The relegation to question ninety shows clearly that moral laws are not the heart of the matter, nor are they the starting-point for ethical deliberation. They are utterly subordinate to and ordered around happiness."