Gratitude is the state of mind of thankfulness. As it is cultivated, we experience an increase in our "sympathetic joy," our happiness at another's happiness. Just as in the cultivation of compassion, we may feel the pain of others, so we may begin to feel their joy as well. And it doesn't stop there.
A Year to Live: How to Live This Year as If It Were Your Last (ed. Harmony, 2009) - ISBN: 9780307561329