‘Rejoice in the Lord’, said St Paul (Philippians 3:1). And he was right to say, ‘in the Lord’. For if our joy is not in the Lord, not only do we not rejoice, but in all probability we never shall. Job, as he described the life of men, found it full of every kind of affliction (Job 7:1-21), and so also did St Basil the Great. St Gregory of Nyssa said that birds and other animals rejoice because of their lack of awareness, while man, being endowed with intelligence, is never happy because of his grief….since life is full of pain and effort, like a state of exile dominated by sin.
But if a person is constantly mindful of God, he will rejoice: as the psalmist says, ‘I remembered God, and I rejoiced’ (Psalm 77:3). For when the intellect is gladdened by the remembrance of God, then it forgets the afflictions of this world, places its hope in Him, and is no longer troubled or anxious. Freedom from anxiety makes it rejoice and give thanks; and the grateful offering of thanks augments the gifts of grace it has received. And as the blessings increase, so does the thankfulness, and so does the pure prayer offered with tears of joy.
~St Peter of Damaskos
Prayerful Life 