There are two imperative verbs which frame Philippians 4:4-9. The first is found in verse 4 and the second is found in verse 8. Look at that first imperative verb with me: “Rejoice in the Lord always. I will say it again: Rejoice!”
Paul is telling us something important: joy is a decision. Joy is not a circumstance, and neither is it a certain situation. No, joy is a decision.
Many times we say that joy is a gift from God. Reflecting upon this, you may wonder, how is joy both a decision and a gift? Let me put it in this way:
Joy is a decision we make that leads to the gift God will give.
In her book Fight Back with Joy, Margaret Feinberg relates how after being diagnosed with cancer, she made a decision to fight the difficulty by choosing joy. She writes:
Practicing defiant joy is the declaration that the darkness does not and will not win. When we fight back with joy, we embrace a reality that is more real than what we’re enduring and we awaken to the deepest reality of our identity as beloved, joyful children of God.
We need to stop waiting for the perfect day in which we can rejoice. We need to stop waiting for the perfect circumstances – at work, in our relationships, in our financial situation – thinking that will enable us to rejoice. No, the perfect circumstances will not give us joy. Otherwise the wildly wealthy or superbly successful would automatically have joy. But we know that is not the case.
No, something different is true. Joy is a choice. Will you decide with me for you right now?
 Margaret Feinberg, Fight Back with Joy (Brentwood, TN: Worthy Media, 2015), 15.