Objection 1. It would seem that joy is a virtue. For vice is contrary to virtue. Now sorrow is set down as a vice, as in the case of sloth and envy. Therefore joy also should be accounted a virtue.
Objection 2. Further, as love and hope are passions, the object of which is "good," so also is joy. Now love and hope are reckoned to be virtues. Therefore joy also should be reckoned a virtue.
Objection 3. Further, the precepts of the Law are about acts of virtue. But we are commanded to rejoice in the Lord, according to Philippians 4:4: "Rejoice in the Lord always." Therefore joy is a virtue.
On the contrary, It is not numbered among the theological virtues, nor among the moral, nor among the intellectual virtues, as is evident from what has been said above (I-II, 57,60,62).
I answer that, As stated above (I-II, 55, 2,4), virtue is an operative habit, wherefore by its very nature it has an inclination to a certain act. Now it may happen that from the same habit there proceed several ordinate and homogeneous acts, each of which follows from another. And since the subsequent acts do not proceed from the virtuous habit except through the preceding act, hence it is that the virtue is defined and named in reference to that preceding act, although those other acts also proceed from the virtue. Now it is evident from what we have said about the passions (I-II, 25, 2,4) that love is the first affection of the appetitive power, and that desire and joy follow from it. Hence the same virtuous habit inclines us to love and desire the beloved good, and to rejoice in it. But in as much as love is the first of these acts, that virtue takes its name, not from joy, nor from desire, but from love, and is called charity. Hence joy is not a virtue distinct from charity, but an act, or effect, of charity: for which reason it is numbered among the Fruits (Galatians 5:22).
Reply to Objection 1. The sorrow which is a vice is caused by inordinate self-love, and this is not a special vice, but a general source of the vices, as stated above (I-II, 77, 4); so that it was necessary to account certain particular sorrows as special vices, because they do not arise from a special, but from a general vice. On the other hand love of God is accounted a special virtue, namely charity, to which joy must be referred, as its proper act, as stated above (here andk22).
Reply to Objection 2. Hope proceeds from love even as joy does, but hope adds, on the part of the object, a special character, viz. "difficult," and "possible to obtain"; for which reason it is accounted a special virtue. On the other hand joy does not add to love any special aspect, that might cause a special virtue.
Reply to Objection 3. The Law prescribes joy, as being an act of charity, albeit not its first act.
Source: Summa Theologica (New Advent)