One is that we have become confused about what joy is, and the second is that we're confused about where it comes from. As a concept, it tends to be used synonymously with happiness , gladness , and pleasure . Listen to the prophet Habakkuk. I have trouble seeing the prophet turning cartwheels of ecstasy over all those disappointments and failings. What he is giving voice to is the distinction that, while things are going badly in his life just now, such circumstances don't interfere with his joy.
Happiness is circumstantial, subject to the shifting vicissitudes of everyday life.
Stimulated or stifled by the stuff that happens to us, happiness can have as much to do with the bad things that are absent, as the good things that are unfolding. Joy, on the other hand, is something altogether deeper, and built of stronger, more resilient stuff. Joy is simply another name for the experience of that rest. Desiring ultimate security, complete validation, eternal love, and constant reinforcement, joy is neither mood nor emotion but the by-product of appropriating the reality of such grace from God. And while such acquisitions might make us happy for the moment, they cannot offer joy.