Rejoice with those who rejoice; mourn with those who mourn. Romans 12:15 NIV
Empathy: understanding what someone else feelings; putting ourselves in someone else's shoes.
"Rejoice with those who rejoice..." Remember a time when you did. A time you got a great surprise, something turned out for the best or someone you love said "I love you" back. Even more, the joy of Spirit-filled moments.
Celebrate the blessing small and large, whatever makes that someone you are with rejoice. Let the Spirit fill your heart and theirs. Remember you own rejoicing but stay in the moment to rejoice "with" not "beside".
"...mourn with those who mourn." When you remember the feeling of grief, acknowledge the feeling but don't fall into it. You're there to support someone else not to go into your past.
People mourn for many reasons, some that might seem small to you but are big for them; and it doesn't matter how long or short the grief lasts. Do not be impatient with those who mourn. Pray they find their way to acceptance of their loss, that the Great Consoler, Christ Jesus, come to their aid.
This is the command to let the Spirit work through you to validate the other person's feelings. To make them aware they do not have to be alone in good times or bad. Most importantly to gently - by your actions - help remind them that our GOD is with them every step of their journey to His Love, through His Son, Jesus Christ, our Lord.