Many see the word "joy" and think of happiness. However, they are two very different things. Happiness is a temporary feeling that shifts and changes as easily as the wind blows. It is often determined by your situation or your mood.
Joy, on the other hand, is a solid foundation that Jesus gives us to stand on.
John 16:24 -- "You haven’t done this before. Ask, using my name, and you will receive, and you will have abundant joy."
By calling on Lord Jesus and maintaining a close relationship with Him, He gives us, not only joy, but abundant joy. It is more than a passing feeling, it is the knowledge that when everything else is falling apart, Jesus is standing in the middle of the ashes holding you up. It is the wisdom to know a bad day does not mean a bad life. Joy is the love that passes between Christ and us and then goes through us to reach out to the world. This is constant, no matter what circumstances you find yourself in, good or bad.
Let's face it, we are not always happy. We sin and make mistakes. We live and work among people who sin and make mistakes. The circumstances are not always favorable.
James 1:2 -- "Dear brothers and sisters, when troubles come your way, consider it an opportunity for great joy."
James does not say "if" trouble comes your way, he says "when." Trouble will come your way. Hard times hit. But those are the times that show your character. Those are the times we need the Holy Spirit to impart the Fruits of the Spirit to us.
Joy does not mean times are always good. It means even while a midst times of struggle, you know who can handle the uncertainty. You know that God has everything under control and that He will take care of you. He will guide and guard you through uncertainties. And He will grant you even more wisdom and experience along the way that will serve the purpose of helping other and advancing God's Kingdom.