John 16:33 'These things I have spoken unto you, that in me ye might have peace. In the world ye shall have tribulation: but be of good cheer; I have overcome the world.'
The world experiences joy and happiness in direct proportion to their circumstances. Bad circumstances produce depression and sorrow, while good circumstances produce joy and peace. That's bondage, and does not have to be the case for a Christian. Our joy is not dependent on things, but rather on the person of Jesus Christ. He is our peace and joy.
The way we take advantage of this joy and peace in the midst of tribulation, is to have our minds and hearts stayed on things above and not on things of this earth. The invisible things of God are eternal while the visible problems on earth are only temporary. All the problems of this life grow very dim when we compare them to the glory of God that is ours through Jesus.
We can rejoice in the Lord always (Phil. 4:4) because rejoicing is an action, not a reaction to our environment. Joy is a gift from God that was given to us at salvation. It was placed within our born-again spirit and it doesn't fluctuate or diminish; it is constant. The Lord has put the joy inside us and we are to 'work it out' by choosing to obey this command in scripture.
We are commanded to rejoice 'IN THE LORD.' Many people are not experiencing true joy because their joy is in their circumstances. That is, they are waiting to rejoice when things in their lives are going good, and that doesn't happen very often. We are suppose to 'rejoice in the Lord always.'
That means we are supposed to rejoice in who the Lord is and what He has done for us. He never changes (Heb. 13:8) and His mercies and compassions are new every morning (Lam. 3:22-23).