NOTE: This is a condensed excerpt from my forthcoming (2014) commentary on Philippians.
In this context Paul’s joy is rooted in the progress of the gospel through the proclamation of Christ. In other words, it is a joy that is rooted in the gospel rather than his personal circumstances. As he sits in Rome under house arrest his personal circumstances are not favorable, but because his joy is rooted in the progress of the gospel it is impervious to the discomfort he is experiencing personally. The present tense of the verb translated rejoice likely portrays Paul joy as a continual experience.
What is our joy rooted in? If we are honest, we find ourselves looking often to our circumstances for joy. When things in our lives are favorable or going our way, we are joyful. But when life takes a turn we do not like, large or small, joy seems like a distant memory. But when our joy is in the gospel of Jesus Christ and its progress in the world, we have an anchor for our joy that will weather even the darkest storms of life. Only in the good news of who Jesus is and what he has done for us will we find the kind of lasting joy that God created us to experience. Everything else will at some point disappoint.
Paul also models for us a prioritization of the progress of the gospel. He is not blind to the selfish motives that some have in preaching the gospel, but he remains grateful that the gospel is still going forth. In this he is a model for us as we observe the advance of the gospel through ministries that we have reason to believe have suspect motives. We can rejoice that the message of the gospel is going forward, even while we retain serious concerns about those taking it forward. Notice that Paul does not say that the motives are unimportant! But he does indicate that he can find joy in the fact that people are hearing the gospel, even though the instrument is flawed. The challenge is learning to rejoice in the progress of the gospel without turning a blind eye to those preaching it for selfish motives.