Happiness depends on situations and circumstances—your health, your financial state, the way your husband or your wife is treating you. Because of those circumstances, happiness fluctuates. Not that there is anything wrong with happiness. I like happiness. But we cannot expect to have happiness all the time.
Joy is different. Joy is a thing of the spirit and depends only on God. In Psalm 43:4, David says, “Then will I go to the altar of God, to God, my joy and my delight. I praise you with the harp, O God, my God” (NIV). In the context of this psalm, David is downcast. He is depressed. Things are going against him. He is unhappy. But he recognizes the solution for unhappiness.
It is to get back in touch with God through his spirit. God does not change. There are no circumstances with God. He is the great invariable, unchanging, eternal source of joy.