I agree with Boanerges, that many times worldly happiness is confused with Christian joy. Christian joy is one of the graces which is part of the Devine Natue we received at conversion. It is a blessed virtue dirived from the love we recieved from God. It does not come and go depending on circumstances, as does happiness, even if happiness may sometimes last a long time.
To know we were chosen by God to serve and love Him, is the greatest honor anyone can experience. That in itself is reason enough for our joy. When bad times come our way, that joy is still there to comfort us. It does not go away. Just like our salvation doesn't stop just because persecution, hate, envy,etc. is heaped on us. It is a premanent fixture in Christian life. Repeating, joy is ours because we are in union (part of) with the creator of the universe.
If by some chance we are not living in the faith, love and obedience, according to Scripture, we may be fooling ourselves and we may not be partakers of God's Devine Nature, given by His grace. Then when good times comes our ours, happiness is the result, and not joy. But:
The Joy of the Lord, which is ours, glories in weakness and tribulation. The apostle Paul displayes this fact in
2nd Corinthians 12:10. "That is why for Christ's sake, I delight in weaknesses, in insults, in hardships, in persecutions, in difficuties. For when I am weak then I am strong." For certain, Paul was the most joyful Christian, next to Christ.
Therefore, let us experience the Joy of the Lord in good times and bad.