In the gospel passage that is read every year for Ash Wednesday, Jesus warns his listeners not to pray or fast or give alms in such a way that others see it. Obviously, this is (in part) because we should avoid doing a penitential action for some sort of reward, or else it isn't really a penance. But I think there is also a deeper sense to what Jesus is saying. For me, penance is not just about doing something or giving something up: it is a call to live in integrity.
To live in integrity means to live in reality. Jesus is warning his listeners to not simply put on their "penance masks" when the pray, fast, and give alms, but for this outward attitude to be a true reflection of a genuine interior disposition. So it means being real with ourselves, first and foremost.
But penance, in my opinion, also means being real (or, if you prefer, "realistic") about the world around us. And this is where Jesus' call to joy-in-penance comes. He tells his disciples, "do not put on a gloomy face". In other words, if we are doing penance properly, the only way to have a gloomy face is to choose to put one on! Real penance, done properly, actually brings joy, thanks to the fact that penance (for it to be real penance) requires us to acknowledge the total truth about ourselves — including the truth that we are sinners! And the acknowledgement of this total truth is the open door to joy.
You may be wondering what I am talking about, and to be honest it is still something I am exploring myself. I can only share one insight to try and explain myself. As I visit patients in the hospital, I have observed something among those who suffer from mental illness: those who are delusional or on the edge of delusions (e.g. schizophenics) almost never laugh. Actually, I have never heard one such person laugh, ever. I've come to realise that it is because laughter is a reaction to the presence of the absurd. Those who suffer from delusions are in some ways disconnected from reality, so they cannot actually recognize the absurd — and so, they never laugh.
Well, we can be pretty absurd sometimes, and the most absurd thing we can do is sin. In my view of things, I expect that the Devil is a very serious individual all the time, while God is very mirthful. Oh yes, God is sorry when he sees us sin, just as we should be sorry for our sins, but at the same time we know we are becoming more and more realistic with ourselves when we can acknowledge how silly and stupid our sins really are.
And it helps us understand why God is so ready to forgive us. Parents, how many times have you seen your kids to something wrong, and you got upset at the time, but later when you thought about it something about the situation was actually pretty funny? Well, we are God's beloved children. Yes, we are constantly getting into trouble and needing correction, but behind the Lord's disapproval of sin is the little smile of his joy — a little smile that turns into a broad gesture of love whenever we turn back to him.