Paul points out in chapter one of Philippians that sharing the Gospel is a source of JOY for himself
and that he also receives JOY in knowing that the Philippian Christians are sharing the Gospel with others.
It's kind of a snowball effect:
I shared with you (JOY),
you shared with others (JOY-JOY),
I'm happy that you are sharing with others (Triple-JOY)
But . . .
He is under house arrest; his chains restricting him to a small house in Rome. He is separated from those who are dear to him. He has been beaten, tortured, imprisoned, shipwrecked. His body must have been racked with constant pain, even on good days. He knows that very soon he will be sentenced to death for doing what THE LORD has called him to do.
Yet, in this state of affairs, he writes a letter whose constant theme is JOY.
His famous quote
"Rejoice in the LORD always! Again, I say, rejoice!"
And why should he.
He knows where
TRUE JOY is found.
Maybe that's why so many of us aren't so joyful. We simply aren't sharing the Good News.