"SYMPATHETIC JOY - The definition is: being happy with someone's fortune/happiness. Sympathetic joy here refers to the potential of bliss and happiness of all sentient beings, as they can all become Buddhas.
The near enemy is hypocrisy or affectation.
The opposite is jealousy, when one cannot accept the happiness of others."

The Buddha taught the following to his son Rahula (from "Old path white clouds" by Thich Nhat Hahn):
"Rahula,
Practice loving kindness to overcome anger.              Loving kindness has the capacity to bring happiness to others without demanding anything in return.                          
Practice compassion to overcome cruelty. Compassion has the capacity to remove the suffering of others without expecting anything in return.
Practice sympathetic joy to overcome hatred. Sympathetic joy arises when one rejoices over the happiness of others and wishes others well-being and success
Practice non-attachment to overcome prejudice. Non-attachment is the way of looking at all things openly and equally. This is because that is. Myself and others are not separate. Do not reject one thing only to chase after another.
I call these the four immeasurables. Practice them and you will become a refreshing source of vitality and happiness for others."  ~from: Views On Buddhism.org  http://viewonbuddhism.org/immeasurables_love_compassion_equanimity_rejoicing.html