In Buddhist philosophy, mudita is the third of the four brahmaviharas, the inner "divine abodes" of loving kindness, compassion, joy, and equanimity that are every human being's true nature. The term mudita is often narrowly translated as "sympathetic" or "altruistic" joy, the pleasure that comes when we delight in other people's well-being rather than begrudge it. But since in practice it's all but impossible to experience happiness for others unless we first develop the capacity to taste it in our own lives, many Buddhist teachers interpret mudita more broadly as referring to the inner fountain of infinite joy that is available to each of us at all times, regardless of our circumstances. The more deeply we drink from this fountain, the more secure we become in our own abundant happiness, and the easier it then becomes for us to relish the joy of other people as well.
We've probably all had moments that have shown us that happiness has virtually nothing to do with the external circumstances of our lives and everything to do with the state of our minds and hearts. We can be drinking margaritas on a Caribbean beach, totally miserable; we can be late for work and stuck in freezing sleet in a traffic jam on the George Washington Bridge, overflowing with bliss.