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.
Your spirit is king.
Once a branch is separated from the vine, it no longer has a life source. Likewise, once we are separated from Christ, we lose our life source and we slowly begin to die.