"There is no greater pleasure than seeing your loved ones prosper" is talking about love maturing into something amazing. When you care enough about someone, they start to become part of you. Suddenly, their welfare is now you're emotional welfare. At a certain point (mostly in marriages), you even care for your loved ones more than yourself—therefore making their prosperity a greater joy than yours.
It reminds me of Jesus Christ, romance, and socialism.
Thanks for posting the question,