All of this means that intentionally and earnestly celebrating the attainments of others amounts to participating in that attainment, karmically-speaking. Rejoicing in one's own virtuous actions, one's Buddhist activities, redoubles the strength of the seeds planted. Seeds bear fruit:
Needless to say, boundless will be the meritsLotus Sutra, Murano translation, p. 268
Of the person who hears this sutra with all his heart,
And expounds its meanings,
And acts according to its teachings.
I encourage you to rejoice in the wholesome deeds and activities of others, and the good qualities of the Buddha, Dharma, and Sangha.
Having heard much of the Lotus Sutra and hopefully taken it to heart, what is to be rejoiced in? Reflect on what you have learned so far: How can you incorporate these teachings into your spiritual practice? Into your everyday conduct with others and on your own?