Because true joy is part of the fruitage of God’s holy spirit. With the help of God’s spirit, we can fully develop “the new personality, ” which includes joy.
(‏ اف ٤ :‏ ٢٤ ؛‏ غل ٥ :‏ ٢٢‏ )‏ وَحِينَ نُنَمِّي هٰذِهِ ٱلصِّفَةَ ،‏ نَصِيرُ أَكْثَرَ ٱسْتِعْدَادًا لِمُوَاجَهَةِ ضُغُوطِ ٱلْحَيَاةِ .‏