Rejoice with them that do rejoice, and weep with them that weep.

People's New Testament

12:15 Rejoice with them that do rejoice, etc. Sympathize with the joys and sorrows of others.

Wesley's Notes

12:15 Rejoice - The direct opposite to weeping is laughter; but this does not so well suit a Christian.

Jamieson-Fausset-Brown Bible Commentary

15. Rejoice with them that rejoice; and weep-the "and" should probably be omitted.

with them that weep-What a beautiful spirit of sympathy with the joys and sorrows of others is here inculcated! But it is only one charming phase of the unselfish character which belongs to all living Christianity. What a world will ours be when this shall become its reigning spirit! Of the two, however, it is more easy to sympathize with another's sorrows than his joys, because in the one case he needs us; in the other not. But just for this reason the latter is the more disinterested, and so the nobler.

Matthew Henry's Concise Commentary

12:9-16 The professed love of Christians to each other should be sincere, free from deceit, and unmeaning and deceitful compliments. Depending on Divine grace, they must detest and dread all evil, and love and delight in whatever is kind and useful. We must not only do that which is good, but we must cleave to it. All our duty towards one another is summed up in one word, love. This denotes the love of parents to their children; which is more tender and natural than any other; unforced, unconstrained. And love to God and man, with zeal for the gospel, will make the wise Christian diligent in all his wordly business, and in gaining superior skill. God must be served with the spirit, under the influences of the Holy Spirit. He is honoured by our hope and trust in him, especially when we rejoice in that hope. He is served, not only by working for him, but by sitting still quietly, when he calls us to suffer. Patience for God's sake, is true piety. Those that rejoice in hope, are likely to be patient in tribulation. We should not be cold in the duty of prayer, nor soon weary of it. Not only must there be kindness to friends and brethren, but Christians must not harbour anger against enemies. It is but mock love, which rests in words of kindness, while our brethren need real supplies, and it is in our power to furnish them. Be ready to entertain those who do good: as there is occasion, we must welcome strangers. Bless, and curse not. It means thorough good will; not, bless them when at prayer, and curse them at other times; but bless them always, and curse not at all. True Christian love will make us take part in the sorrows and joys of each other. Labour as much as you can to agree in the same spiritual truths; and when you come short of that, yet agree in affection. Look upon worldly pomp and dignity with holy contempt. Do not mind it; be not in love with it. Be reconciled to the place God in his providence puts you in, whatever it be. Nothing is below us, but sin. We shall never find in our hearts to condescend to others, while we indulge conceit of ourselves; therefore that must be mortified.