Objection 1. It would seem that the spiritual joy that results from charity is compatible with an admixture of sorrow. For it belongs to charity to rejoice in our neighbor's good, according to 1 Corinthians 13:4-6: "Charity... rejoiceth not in iniquity, but rejoiceth with the truth." But this joy is compatible with an admixture of sorrow, according to Romans 12:15: "Rejoice with them that rejoice, weep with them that weep." Therefore the spiritual joy of charity is compatible with an admixture of sorrow.
Objection 2. Further, according to Gregory (Hom. in Evang. xxxiv), "penance consists in deploring past sins, and in not committing again those we have deplored." But there is no true penance without charity. Therefore the joy of charity has an admixture of sorrow.
Objection 3. Further, it is through charity that man desires to be with Christ according to Philippians 1:23: "Having a desire to be dissolved and to be with Christ." Now this desire gives rise, in man, to a certain sadness, according to Psalm 119:5: "Woe is me that my sojourning is prolonged!" Therefore the joy of charity admits of a seasoning of sorrow.
On the contrary, The joy of charity is joy about the Divine wisdom. Now such like joy has no admixture of sorrow, according to Wisdom 8:16: "Her conversation hath no bitterness." Therefore the joy of charity is incompatible with an admixture of sorrow.
I answer that, As stated above (1, ad 3), a twofold joy in God arises from charity. One, the more excellent, is proper to charity; and with this joy we rejoice in the Divine good considered in itself. This joy of charity is incompatible with an admixture of sorrow, even as the good which is its object is incompatible with any admixture of evil: hence the Apostle says (Philippians 4:4): "Rejoice in the Lord always."
The other is the joy of charity whereby we rejoice in the Divine good as participated by us. This participation can be hindered by anything contrary to it, wherefore, in this respect, the joy of charity is compatible with an admixture of sorrow, in so far as a man grieves for that which hinders the participation of the Divine good, either in us or in our neighbor, whom we love as ourselves.
Reply to Objection 1. Our neighbor does not weep save on account of some evil. Now every evil implies lack of participation in the sovereign good: hence charity makes us weep with our neighbor in so far as he is hindered from participating in the Divine good.
Reply to Objection 2. Our sins divide between us and God, according to Isaiah 59:2; wherefore this is the reason why we grieve for our past sins, or for those of others, in so far as they hinder us from participating in the Divine good.
Reply to Objection 3. Although in this unhappy abode we participate, after a fashion, in the Divine good, by knowledge and love, yet the unhappiness of this life is an obstacle to a perfect participation in the Divine good: hence this very sorrow, whereby a man grieves for the delay of glory, is connected with the hindrance to a participation of the Divine good.
Source: Summa Theologica (New Advent)