We take happiness for granted. It's like we think we deserve it. But we don't. Ultimately we deserve hell. Christ changed that by suffering the wrath of God in our place so that we can experience the joy of eternal life.
Happiness is temporal but joy is found in the eternal.
So we should be joyful...when? Only when we remember what lies ahead (in heaven)? Or how about when things are going our way? Rejoice in the Lord every-once-in-a-while? No. "Rejoice in the Lord always: and again I say, rejoice." (Phil. 4:4)