Someone mentioned this to me, since I am not very familiar with Buddhism in general and I thought it might be worth discussing here.
Mudita is one of the Brahma-Viharas, or "four immeasurables" of Buddhism.
Mudita is roughly translated as empathetic, or unselfish joy. In a sense, sharing in the happiness, well being and enlightenment of others, when there is no direct benefit. It is said to be the most difficult of the four immeasurables, but also one of the most rewarding.
The idea is that mudita will strengthen our capacity for joy and allow us to achieve happiness, despite any personal feelings of insufficiency and even when facing personal tragedy. In a deeper sense, if we share in the well being of others, and others do the same, then the overall outcome is positive-sum.
The "near enemy" of mudita is exuberance, or the over-attachment of joy and sadness to events in our personal lives.
The "far enemies" of mudita are jealousy and envy, the resentment of the joy of others and the good fortune that they have received.
Some suggestions about cultivating mudita are provided here. But I am not so sure about the first suggestion "Fake it till you make it", since the point is to cultivate real joy.