NOTE: This is a condensed excerpt from my forthcoming (2014) commentary on Philippians.
When Paul prays for the Philippians, he does so with joy. The related ideas of joy (chara) and rejoicing (chairō) are a major theme in Philippians. In contrast to happiness, which is rooted in circumstances, joy is a deep-seated confidence and delight in God and his promises that transcends circumstances.That is why although Paul is in Roman custody awaiting trial he can rejoice in the progress of the gospel (1:18, 25; 2:17-18) and call the Philippians to rejoice (2:18; 3:1; 4:4) despite suffering persecution (1:27-30) and facing internal conflict (4:2-3). Paul’s prayers for the Philippians do not spring from dutiful obligation but from the joy he finds in God and the Philippians’ fellowship in the gospel.
Are your prayers for others marked with joy?
 “Joy for the biblical writers is not primarily a mood or an emotion; it is not dependent on success or well-being or outward circumstances” (Bockmuehl, Philippians, 59).