Joy is mentioned 6x in Philippians and rejoice is mentioned 10x. Certainly one of the reoccuring themes of Philippians is this matter of rejoicing. Rejoice means to be well or to be glad. Understanding the background and circumstances of Paul in writing this epistle should help us to see that Christians can even in difficult times 'rejoice.' I think a key to rejoicing is the object of our joy. So that if we joy in the Lord, then we can rejoice even in hard times. Our source of joy is the Lord. And when things change, we know that God does not change. We know He is in control and is always desiring and working out that which is best for us. God is a loving Father, and since by faith we are His children, He will care for us as a father does his children, and even better than that, because He is far more excellent in His character than we are towards each other.