Christians should display more joy in their everyday lives than any group on earth; sadly this is not always the case.
Our days should be filled with joy, not walking around wearing mugs that would blend right in with the wanted posters in the Post Office. Christian joy should translate to smiles and friendliness, not frowns and gloominess.
Christians are not excluded from many of the same problems that weigh-down all humans, we often find ourselves in the same morass as unbelievers, and bear many of the same burdens, sadness, and guilt. Thankfully, those in Jesus have an ever present help that we can bet our lives on, and this alone should cause us to overflow with joy.
The first century church had a ton of joy even during one of the worst periods of Christian persecution in history. A few of our "church" tribes believe they have captured the essence of the early church, but seem to have neglected to restore the joy of being a Christian.
As His children, we realize God loves and protects us, yet we often mope around looking in every nook and cranny for the negatives, while the obvious blessings that surround us escape our notice. Counting our daily blessings should give us all the incentative we need to express ourselves in a joyful manner.
Like most Christians, I fail miserably at times to possess the joy that Jesus gives, and certainly don't always reflect a joyful image as I should, but one thing I have discovered; if I start out each day with a smile and a kind word (even if I'm dying inside,) people respond with a tidal wave of kindness and joy.
Perhaps if we spent a little time in quite contemplation we might realize the blessings we reap daily as children of God; our sins are forgiven, we have a God who listens to us, we possess the Spirit of God within us, we are blessed with a Christian community, and finally we have the promise of eternal life in the presence of God...surely this is enough to promote an occasional smile.