Father Dwight blogs on it here.  Joy is one of those things we are often urged to have, but rarely told how to go about having it.    I’d describe myself as partly-joyful.  Because part of the time, this happens to me:

"Joy springs up from the depths of a heart that has been truly converted by the power of the resurrection.  That is the best way to describe joy: it is a heart raised up and being raised up and forever being raised up."

As mine was in response to that prayer request I mentioned the other day.  "Ask and you shall receive." The trick for me is remembering to ask, and then, after, not tossing out that which I just received.

An internet friend shared recently how, when she was coming into the church, she heard a lot of talk about humility, but no one explained what it actually was.  Finally she asked her priest, and he told her, "Humility is simply telling the truth.  About yourself, about others, and about God."

Joy would be, then, the natural response to that truth.   We are dead in our sins, but God saves us.  For me it is the sins against humility — not just pride, but despair, doubt, anger — well, just about most of the sins — that are the kill-joys.   The sacraments of reconciliation and holy  communion, on the other hand, are the build-joys. 

Catholics are privileged in that way, because we have not just a promise to cling to, but a promise that is delivered upon, week after week after long, difficult, desperate week.  When we come across a joyful protestant (and there are many), we are coming across a good shoulder-shaking.  To hear the Lord say "Your sins are forgiven", to attend to Him and receive Him in his real, physical presence — the mere hope of one day experiencing these things, animates the most fervent, genuine, hand-raising, eyes-weeping protestant praise and worship services.  Rightly so.  And for catholics, they are ours to have, now.