Why can a Christian have joy no matter what happens in this life?
Joy is an inner reality produced by the Holy Spirit's indwelling and by what God has done on the believer's behalf. Regardless of what happens in this life, the Christian has eternal life and God's prepared promises. This precipitates real joy, while happiness depends only on favorable circumstances and cannot sustain the believer in trials.
Les explains that joy comes from within as a result of the Holy Spirit indwelling the believer and as a result of what God has done on our behalf. Because the Christian has eternal life and the promises of all things God has prepared, he or she can have joy unspeakable regardless of what may happen in this life. That inner reality precipitates real joy. Happiness, by contrast, is the result of circumstances—being healthy, having bills paid, money in the bank, a decent car, or a nice home. Those things are only circumstantial and have nothing to do with the inner life. So whether one has money or is in trial and tribulations, one can still have joy, though not necessarily happiness.