When a friend rejoices, the worldly person is envious of the reason for the joy. When there is sadness, the worldly person moves away, unable to deal with the pain. In Christ, however, there is solidarity.
Rejoice with those who rejoice, weep with those who weep.
Romans 12:15 NET
In order to fulfill the order above, it’s necessary to enter into the other’s experience, listen without commentary or counsel, accept the person in his condition. It means participating in his life, instead of competing or distancing oneself. It is being present, being a friend rather than a critic.
With other brothers and sisters in Christ, it’s possible to have such intimacy, but not with those outside of Christ. There may be a friendship of sorts, but conflicting goals prohibit the communion which faith creates. The world tries unsuccessfully to imitate Christian love, but only in Christ, himself God who became man, is this solidarity real.