And they kept the Feast of Unleavened Bread seven days with joy, for the Lord had made them joyful and had turned the heart of the king of Assyria to them, so that he aided them in the work of the house of God, the God of Israel (Ezra 6:22).
The fulfillment of God's Word results in joy. It is the inevitable product of trusting obedience to God. Consider what's happening in the context:
God freed from them from exile.
God delivered and protected them from their enemies.
God blessed them with abundance.
His gracious mercy, their repentance, and worship culminate in joy.
It's a glimpse of Heaven. This is how God "made them joyful." This is how God makes His people joyful still.
Christians derive joy from the assurance of God's promise of Heaven––"He who promised is faithful" (Heb. 10:23), thus Christians derive joy from doing the things which secure that promise––"Make your calling and election sure" (2 Pet. 1:10).
Joy begins here and now, but there is greater joy to come in eternity where we will finally and fully experience the fulfillment of God's promises.
There's a song we often sing called "When He Comes In Glory By and By"––the refrain goes like this:
How sweet! How sweet!
When He comes in the sky!
What joy! What joy!
When He comes in glory, by and by.
Think of it: What will it be like to be unburdened from the daily grind to survive? To be unburdened from our flesh––not just from failing health and aging, but also from a moral fallibility standpoint, wherein there will be no more temptation? To be unburdened from the cares of this life––the threats, the taxes, the bills, the fears, the tragedies, the loss, and the mental anguish caused by the evil around us? In a moment, it will all be gone. "How sweet 'twill be," indeed! Think about what we discuss with one another and with our families around the table: the troubles, the conflicts, the sadness, the disappointments, the frustrations––whether we’re talking about things close to home, things in society, or the national level––it is all tainted with the consequences of sin. Yet God will tear it all down in the end, He will right all the wrongs, this should give us relief and joy. Such joy is grounded in the promises and truth of God's Word, and such joy can be yours only to the extent you trust and obey that Word. The joy Jesus offers is not drug induced, naïveté, artificial optimism, or a perpetual emotional high. It's REAL and exists in defiance of any circumstance, because it does not reside in circumstances or positive attitudes toward life. Joy reigns in the heart only when Christ is Lord of life. Joy is always in the Lord. "Rejoice in the Lord always. I will say it again: Rejoice!" (Phil. 4:4) In the final analysis, there are no reasons for rejoicing without the Lord, but with Him there's no end to such reasons.