Left on our own, there is no way we can look at our circumstances or Paul look at his wrongful imprisonment and write about joy. But with God, even though Paul was externally in chains, internally he was free and partaking of the joy of the Lord. He was allowing himself to receive the fruit of joy from the Holy Spirit and he was choosing to let it overflow through him despite his present circumstances.
"We have come into an intimate experience with God’s love, and we trust in the love he has for us. God is love! Those who are living in love are living in God, and God lives through them."