From The Practice of Godliness by Jerry Bridges:
So the choice is ours. We can be joyless Christians, or we can be joyful Christians. We can go through life bored, glum and complaining, or we can rejoice in the Lord, in our names being written in heaven, in the hope of an eternal inheritance. It is both our privilege and our duty to be joyful. To be joyless is to dishonor God and to deny His love and His control over our lives. It is practical atheism. To be joyful is to experience the power of the Holy Spirit within us, and to say to a watching world, "Our God reigns."
Joy is a fruit of the Spirit. It is a result of His work, but it is also something we must do. We must, by His power, rejoice. This is part of the practice of godliness.