Contemporary English Version (CEV)
Let's imagine you win the lottery tomorrow -- you'd have tremendous joy, right? That would last for a while, but in time, be it weeks or months or years, studies show that your level of joy would be set at such a point that it would take even greater news to bring you the same exceeding jolt of joy. It becomes even harder, because we compare news to the last big event in our lives.
In other words, it's hard to sustain joy when we base it on the things and events in the world, because we need more and more. We ask the world for more and more, and eventually we learn that our appetites can't truly be sated by the things in this world, because we were made for more.
True joy, the lasting and eternal joy that comes from Christ, can sustain and amaze us day after day, because the joy we find in Christ is deeper than our minds can fathom. We cannot plumb the depths of Christ and his amazing love, so we can truly lose ourselves in them, and the more we grow spiritually, the greater the depth of understanding and the deeper sense of awe we have. God loves you, and his love is the only thing in this world that can build sustaining joy.
If we base our moods on the happenings in the world around us, we will eventually find ourselves miserable, because they swing back and forth. If, however, we root ourselves in the life-giving love of God, we will discover a joy that knows no end.