This phrase is an invitation to the mature Philippians to realign their thinking orientation. A manifestation of whether we have reached maturity is whether we change attitudes and actions. Paul challenges our attitudes. If we keep tabs on the making of our lives, we can measure our growth. Joy is not happiness.
Happiness depends upon circumstances.
Joy is an inner animation of soul regardless of circumstance. Joy is independent of circumstance. If circumstances are positive, they are happy. If not, then they go down with the circumstance. Their lives undulate like a roller coaster.