Lots of things in this world - circumstances, people, events - can cause our happiness in life to take a hit.
Paul wanted the Philippian church to know that he had gone through it all - yet he still chose joy.
What is Paul's goal? Why does he do why he does despite it causing him so much greif? For him it is all for God's glory - not just here on earth but in the new creation too! His entire life was to be for the praise and glory of God. What's more is that he wants us as children of God to do the same. To be lifted up in joy and praise towards God.
Paul finds his joy in the death and resurrection of Christ. Even the best things in the world are nothing compared to what knowing Jesus and having Him as our saviour brings. If Paul's greatest joy is in Christ then his reward is being clothed in the righteousness of God so that his life can be all for God's praise.
What does living with this joy look like?
Paul thanks God for all the believers with joy (v4). He thanks God for the fact that they join him in praising God and spreading the gospel alongside Him. As Paul looks at those believers, he sees a glimpse of that day when all will glorify God. He knows God will continue His work to completion and that brings Paul so much joy and thankfulness.
Paul chose to be prayerful, and he focussed his prayers on having a huge crop of new believers. He prayed for a harvest of righteousness. He saw the believers doing great things in the name of God and does he prayed for more of it, and the glory that it will bring in the new creation.
The love that causes such outpourings of righteousness comes from God Himself. That love enables us to know what it means to honour and glorify God.
We should be praying like Paul. We should pray for a huge harvest as a result of the outpouring of love and righteousness from knowing God and His love.