Being children of God, how can we be sad? Sadness is the end product of selfishness. If we truly want to live for God, we will never lack cheerfulness, even when we discover our errors and wretchedness. Cheerfulness finds its way into our life of prayer, so much so that we cannot help singing for joy. For we are in love, and singing is a thing that lovers do.
(http://www.opusdei.org.in/sec.php?s=310)
