John Piper, from his book, When I Don't Desire God (free download):
A few days ago I called my eighty-five-year-old father and said, "Daddy, I am writing a book on how to fight for joy. What one thing comes to your mind from sixty years of ministry as to what Christians could do to increase their joy?" Almost without hesitation he said, "Share their faith." Joy in Christ thrives on being shared. That is the essence of Christian joy: It overflows or dies (p. 227).