Joy, what is joy? Joy is found where Christ is found. Christian life is built upon Christ, which means that joy is in all that we believe.
Joy in liturgy and worship, joy in vocation, and yes even joy in suffering.
What more can we do before God that is greater than these two services — the services of prayer and thanksgiving? In the first place, one must hear the Word, which is given to us by God. Here we do nothing, but we only take hold of what has been offered. In the second place, one must pray and implore God’s help after the Word has been heard and taken hold of, and after getting this help one must give thanks and offer sacrifice. But we pray not only with the mouth or the voice but also with sobbing of the heart, with all our strength and members. This is prayer without ceasing. — Martin Luther, AE 5:357
True and only joy is found where Jesus is found. Jesus is found in His Word, His Means of Grace! A forgiveness of sin, extended to all believers! What a joy it is to take comfort in this grace from the LORD.
Then go joyfully to your work, singing a hymn, like that of the Ten Commandments, or whatever your devotion may suggest. — Martin Luther, (Small Catechism, Daily Prayers)