Some of the Lord's commandments to us are not natural to us as human beings. Obedience requires going against our natural inclinations. The commandment to rejoice with those who rejoice and mourn with those who mourn is one that IS natural, but He calls us to go beyond our natural, normal inclinations. Any human, regardless of faith, will normally rejoice at a friend's wedding and mourn with a friend who is experiencing the death of another loved one. If that were all that was required of us, it wouldn't need to be mentioned. But as Christians we need to show that kind of love toward those we are not naturally inclined to love. Everyone the Lord puts in our lives should receive this level of care from us, regardless of who they are.
Is there anyone in my life who I am inclined to be jealous of, to dislike, to find annoying? I still need to desire the best for them, to rejoice when they receive it, and to mourn with them when they experience pain.