Phil. 1:9 I pray that your love will overflow more and more, and that you will keep on growing in knowledge and understanding. 10 For I want you to understand what really matters, so that you may live pure and blameless lives until the day of Christ’s return. 11 May you always be filled with the fruit of your salvation—the righteous character produced in your life by Jesus Christ
Love, Agape love brings joy. The impact of a man and woman who Loves, is joy. As one gives of himself or herself to God, as one gives himself or herself to other believers, joy, this person will be filled with overflowing joy.
Paul gave of himself and thus Paul is able to express his joy even though he is in prison. Life was not about Paul, it was all about God.
Phil. 1:3 Every time I think of you, I give thanks to my God. 4 Whenever I pray, I make my requests for all of you with joy, 5 for you have been my partners in spreading the Good News about Christ from the time you first heard it until now. 6 And I am certain that God, who began the good work within you, will continue his work until it is finally finished on the day when Christ Jesus returns.
Paul had given himself to God and to them self-sacrificially . So he was filled with joy.
He was filled with joy for he had been receiving from them financial help.
He was filled with joy for he had been receiving from them their active participation in the ministry, sending him people to come and help with the work.
He was filled with joy for he had been receiving from them support in prayer.
So Paul is confident of God, that God will continue to work within them, making them more like Christ, because they by their actions have shown they Love God, God is their 1st priority in life.