Romans 12:15

‘Rejoice with those who rejoice; weep with those who weep.'

The Christian should be an expert at getting alongside people in order to share with them their joys and sorrows. Thus he will share in people's rejoicing, and will feel for the miserable in their misery. This is not an excuse for revelling, even though it was common practise to share in people's joys by feasting with them. It is rather expressing the importance of entering into people's feelings, whether cheerful or otherwise. The idea is to share with them in their inner feelings. Compare Job 30:25, and see 1 Corinthians 12:26 where it specifically has Christians in mind. The idea of weeping with those who weep was of course commonplace at funerals, and was encouraged by the practise of having professional mourners. But Paul is applying it to the sorrows of everyday life. The idea here is of expressing consideration and concern for others, and entering into their feelings.