They will never be satisfied as long as they have achieved less than someone else or are less than perfect. In contrast to this dreamland living, Christianity offers joy. Instead of a guaranteed plan for success, Jesus offers victory over the world and its values. God doesn't measure us by the success we attain in this life. He will love us in Christ even if everything we attempt for him ends in total failure.
Happiness depends on the external.
Some situations, things and people make us happy. If those situations, things or people change for the worse, we may no longer be happy. Even the most mature and sensible people cannot maintain happiness at all times. Joy comes from within and is based not on external circumstances but on internal assurance and confidence in Christ. In the five verses following his command to rejoice, Paul offers practical ways to allow joy to affect our attitudes and actions.