Just as, when a person with a burning thirst is very dry and has been parched for a long time, to happen upon a fountain of cold water is a very happy and most welcome occurence, so too, since we are overwhelmed by sadness and weighed down by the burden of numerous tribulations, the refrain of the prophet's song today rouses us to joy. Come, it says, let us rejoice in the Lord. (Ps 94.1a) Come where? Or from where? From yourself, man, into yourself, where not a change of location, but a conversion of your attitudes casts out adversity, puts sadness to flight, dispels dispair, drives away distress, and within the residence of a sincere heart prepares an eternal dwelling of divine gladness.
Then, what has become of: Blessed are those who mourn (Mt 5.4) and Woe to you who laugh? (Lk 6.25) Clearly, blessed are those who mourn in the world, and woe to those who laugh in the world, but blessed are those who rejoice in the Lord, and who gain no happiness from robbery, from deceit, or from the tears of their neighbors. Come, let us rejoice in the Lord. The one who by word, action or deed rejoices not in himself but in his Creator rejoices in the Lord. Come, let us rejoice in the Lord. The one for whom God is always his only and complete happiness rejoices in the Lord. (St Peter Chrysologus, "Sermon 46: On the Ninety-fourth Psalm" in The Fathers of the Church 109.178)
Note: The picture is of the ceiling of the baptistery at the Cathedral in Ravenna where St Peter Chrysologus served as bishop.