John Chrysostom

"Rejoice with them that do rejoice, and weep with them that weep." Since it is possible to bless and not to curse, and yet not to do this out of love, he wishes us to be penetrated with the warmth of friendship throughout. And this is why he goes on in these words, that we are not only to bless, but even feel compassion for their pains and sufferings, whenever we happen to see them fallen into trouble. …

Homily on Romans 22 ↗

Theophylact of Ohrid

A great spirit is needed not only to not envy, but also to rejoice with those who rejoice. This shows a soul completely free from envy, and it means much more than to weep with those who weep; because nature itself, which draws us to compassion, impels us to weep. Thus, he teaches us both the one and the other, that is, both compassion and a disposition free from envy toward all, even toward persecutors.

Commentary on Romans ↗

Thomas Aquinas

Then when he says: to rejoice with those who rejoice, he teaches about concord: first, he sets out nature of concord; second, the obstacles to it, at not minding high things. Concord can be considered in two ways. In one way, as regards the effect it produces in reacting to good and evil. In regard to good it makes one rejoice in the good of others; hence he says: rejoice with those who rejoice: I am glad and rejoice with you all (Phil 2:17). But this refers to rejoicing with one who rejoices in the good. …

Commentary on Romans ↗

Bernard of Clairvaux

The mouth of Paul is a mighty and unfailing fountain, ever open to us all; and as I have often done before, so now too I draw from its resources in my attempt to interpret the breasts of the bride. "Rejoice," he said, "with those who rejoice, and be sad with those who sorrow." In these few words we find the description of a mother's affections, because she shares both health and sickness with her children. She cannot avoid being conformed to them in the depths of her being in these experiences. …

Sermons on the Song of Songs, Sermon 10 ↗