Joy is not simply your personal, egoistic happiness. Nor is it making others happy at the expense of your own happiness. You and others delighting together, you and others becoming happy together–this is the Mystic Law and the wondrous thing about our realm of kosen-rufu. Nichiren states, “Joy means that both oneself and others have wisdom and compassion” (Gosho Zenshu, p. 761).
from sgi.org