Joy is the response to something experienced as good, and it invites rest in that good. The final piece of knowledge we need for real joy is self-knowledge, namely, the knowledge that can provide the foundation for humility. In Joseph Pieper’s words, “Humility is man’s estimation of himself according to truth. And that is almost all there is to it.”

Joy depends on our knowing the truth about ourselves, I think, for at least four reasons. Knowing who and what we are is helps us to: a) cultivate gratitude; b) not think about ourselves too much; c) appreciate the magnificence of other things; d) resist the urge to feel as though our salvation depends upon ourselves.

First, gratitude. Gratitude and a sense of entitlement are diametrically opposed. Have you ever heard someone say “Thank you!” after his insistent demands have finally been met? It doesn’t sound like gratitude – it sounds more like “Well, it’s about time!” You’re not grateful for what you think you’re owed. Gratitude is due to what is given gratuitously. It’s not due to what you claim by right.

In fact, you don’t have any absolute rights. Not before God. Because in your native state, you don’t even exist. You’re a sliver of non-being that God took notice of and brought to life – and non-being has no claims on being. Remember Our Lord’s words to Catherine of Sienna: “Do you know, daughter, who you are, and who I am? If you know these two things you will be blessed. You are she who is not; whereas I am He who is.”

And not only are you absolutely dependent upon God for your existence, you are a sinner! Apart from God, you’re nothing, and then when God lets you participate in being, what do you do? You turn away from Him, you insult Him by preferring to slide back into evil and sin, the shameful non-being of privation. 

You aren't owed anything, so everything is a gift. 

That brings us to the second way self-knowledge causes joy: it encourages us not to think too much about ourselves. We aren’t a big deal, so we shouldn’t think a great deal about ourselves. Narcissism doesn’t just mean thinking a lot of oneself, it also means thinking about oneself a lot. That’s disproportionate to the truth – you should think the most about the most important things, and you yourself aren’t one of them. So humility doesn’t mean constant self-disparagement: as C.S. Lewis says, if you meet someone who’s really humble, you’ll notice that person “will not be thinking about humility: he will not be thinking about himself at all.” That’s as it should be. Satan wants us to think more about ourselves than about anything else, for the simple reason that it’s not good for us and it makes us miserable.

When you know you’re little, you appreciate the magnificence of other things, you refocus your attention on the grandeur of the world beyond yourself. Chesterton tells the story of two boys: one became a titanic giant and the other became as small as a grasshopper. The one who became a giant became bored with everything – the wonders of the world were all toy-sized and unimpressive. The one who became tiny was able to have astonishing adventures everywhere. He appreciated everything, now that he saw how big and rich it all was, but to do that he had to become small first.

Humility is simply a sense of scale, and because you no longer dominate every scene, you can appreciate things for what they are. But it also takes the pressure off.You’re not in control, you’re not the major mover in the universe. God is. Which also means you can take peace in leaving your very salvation up to him.

This was the secret of St. Philip Neri, one of the most famously joyful of all the saints. It’s recorded that one time he gave a very delightful illustration of how to be miserable and how to be happy: