St. Thomas Aquinas: "4 Characteristics of True Joy"
The Apostle touches on four characteristics of true joy;
1. It must be right, which happens when it concerns the proper good of man, which is not something created, but God: but for me it is good to be near God; I have made the Lord God my refuge (Ps 73:28). Therefore, it is right, when there is joy in the Lord; hence he says, in the Lord: the joy of the Lord is your strength (Neh 8:10).
2. It is continuous; hence he says, always, rejoice always (1 Thess 5:16). This happens when it is not interrupted by sin, for then it is continuous. But sometimes it is interrupted by temporal sadness, which signifies the imperfection of joy. For when a person rejoices perfectly, his joy is not interrupted, because he cares little about things that do not last; that is why he says, always.
3. It should be multiple; for if you rejoice in God, you will rejoice in his Incarnation: I bring you good news of a great joy, which will come to all the people; for to you is born this day in the city of David a Savior (Luke 2:10); and in your own activity: when justice is done, it is a joy to the righteous (Prov 21:15); and in your contemplation: companionship with her has no bitterness (Wis 8:16). Again, if you rejoice in your good, you will be prepared to rejoice in the good of others; if you rejoice in the present, you are prepared to rejoice in the future; hence he says, again, I say, rejoice.
4. It should be moderate and not flooded with pleasures, as happens in worldly joy; hence he says, let your modesty be known to all men. As if to say: your joy should be so moderated that it will not degenerate into dissoluteness: the people continued feasting in Jerusalem before the sanctuary (Jdt 16:20). He says, be known to all men, as if to say: your life should be so moderate in externals, that it offends the gaze of no one; for that would hinder your manner of life.