William Burkitt on Romans 12:15

The next duty required, is Christian sympathy and mutual affection between brethren, both in prosperity and adversity, to rejoice in the one, and to mourn together in the other, as being members of the same body. Teaching us,that it is a Christian's duty to rejoice in those good things, whether inward or outward, which befall his brethren; and also to mourn and lay to heart all those afflictions and sorrows, whether inward or outward, which come upon them.

But, Lord! how far are they from this duty, who, instead of mourning for the sufferings of others, are glad at calamity, rejoice at the downfall of others!

O, help us to lay the troubles of others to heart, when we ourselves are freest and farthest from trouble: let us weep with them that weep, and rejoice with, &c. The gospel acquaints us with the pity of God towards us, and presseth us to pity one another.