I thank my God each time I think of you and when I pray for you, I pray with joy. Philippians 1:3-4
In Acts, we saw how the apostles used to establish a community, a church in every city where they had proclaimed the Gospel. They did not leave without having established a Council of leaders.
Paul was in prison when he wrote this letter to the Philippians to give them news about himself and ask the community which always showed him the most love, thus showing his great confidence in them.
He longed to see them but he was being imprisoned. So he could only think of them and as he remembered them, he gave thanks to God for them. And each time he remembered them, he prayed for them and prayed with joy. Paul could not forget all that they had shared with him in the service of the Gospel. The good work which God had began in them.
God who began such a good work in you, will complete it in the day of Christ Jesus was the hope of Paul expressed in several of his letters. He will keep you steadfast to the end, and you will be without reproach on the day of the coming of our Lord Jesus Christ. (1 Corinthians 1:8)
Paul loved them so dearly that he prayed that their love may lead them each day to a deeper knowledge and clearer discernment to have a good criteria for everything. As to be pure of heart and filled with the fruit of holiness, which comes through Christ Jesus, for the glory and praise of God.
It is not happy people who are thankful, it is thankful people who are happy.