Paul encourages the Philippians to “rejoice in the Lord always” (Phil. 4:4). Is he talking about happiness? Most people do want to be happy. Happiness is the good feeling we have when everything goes our way – when our football team wins a big game, when we head home for the holidays, when several inches of slow rain soak the ground after a dry summer, or when we dip up a big bowl of Blue Bell ice cream. Happiness mainly depends on happenings, on present circumstances.
Godless people will do almost anything to grasp after the elusive feeling of happiness. They drink to excess, abuse drugs, 
overspend, engage in destructive sexual behavior, or work endless hours to acquire more stuff. But “rejoice in the Lord always” doesn’t mean “be happy all the time.” Happiness and unhappiness can’t exist together. Joy and sorrow can and do.
Consider the Lord’s own experience. Predicting the suffering of Jesus, Isaiah said that “he was despised and rejected by men; a man of sorrows, and acquainted with grief” (Isa. 53:3). The “man of sorrows” anticipated the cross with a heavy heart. In Gethsemane he prayed “in an agony,” and “his sweat became like great drops of blood falling down to the ground” (Luke 22:44). And yet, the Bible speaks of “the joy that was set before him” as “he endured the cross” (Heb. 12:2). Was Jesus happy in his sadness? No. He rejoiced in his sorrow.
Happiness is a feeling, but joy is an attitude. “Rejoice in the Lord always” is a commandment because rejoicing is something we can choose to do. No one can decide to feel happy when he is feverish and nauseated. But a sick person can choose to rejoice in the Lord—to thank God for his blessings, to trust in his care, and to anticipate the glorious day when “he will wipe away every tear” (Rev. 21:4).