A.T. Robertson Commentary Romans 12:15

"Rejoice with them that rejoice; weep with them that weep." — Romans 12:15 (ASV)

Rejoice (χαιρειν). Present active infinitive of χαιρω, absolute or independent use of the infinitive as if a finite verb as occurs sometimes (Robertson, Grammar, pp. 1092ff.). Literally here, "Rejoicing with rejoicing people, weeping with weeping people."