Joy and happiness are not the same things. Joy is superior, and includes elements of happiness. But happiness does not include joy. Happiness is strictly reactionary. You can’t choose it. You simply experience it. Joy, on the other hand, is both willful and reactionary.
When the sky is blue and cloudless, and there’s a cool breeze, and you have the day off, and you find some extra cash, and the problem you were having yesterday somehow just magically melted away, and your spouse makes your favorite meal and serves you with a genuine smile, then you will likely experience a potpourri of good feelings. They might be thrilling feelings, or warm feelings, or feelings of jubilance, or gladness, or delight, or cheer. We call that mishmash happiness. It’s a pleasing condition. But it’s also fragile and fleeting. It lasts only as long as the circumstances, and it cannot be enjoyed in the midst of sadness or pain. In fact, a bit of sad news can shatter that mishmash into a thousand unconnected pieces and suck all the happiness right out of your body so utterly completely that you will forget that you were ever experiencing it. You can savor happiness, but you cannot really choose it.
Joy on the other hand is both a choice, and a reflex. When you have lived the fullest life with a loved one, and that loved one goes home to be with the Lord, you will feel loss, pain, and sadness. You won’t be able to stop the flood of those feelings. But as you look back on a life you shared with that person, you won’t be able stop the feelings of delight, and warmth, and gladness over your shared victories and experiences. In such a situation, sadness and warmth coexist in a reflexive feeling that we call joy. We also have the ability to choose joy. When we make the conscious choice to focus on what God has done for us, what he is doing for us, and especially what he has promised to do for us, the inevitable result will be a feeling of warmth, hope, and delight that is rightfully called joy. That joy can exist even while we are suffering, being persecuted, experiencing torment, or feeling sadness. That joy comes in knowing that for the lover of God, all pain is temporary, and fullest joy is eternal and permanent. Its highest pleasure comes from the anticipation (and eventual fulfillment) of being united with the highest person for whom you were made and purposed from the beginning — God. This is why Paul commands us with double emphasis to rejoice always. Such a choice brings a supernatural peace that is beyond all understanding (Philippians 4:4-9).