I'm convinced a Christian should be the most joyous person on earth even if some of us aren't! Jesus promised His followers, "Ask and you will receive, and your joy will be complete" (John 16:24). The Bible also says that one of the main things God wants to produce in our lives by His Spirit is joy (see Galatians 5:22-23).
Why should Christians be joyful? It isn't because they are blind to the world's problems and don't realize the world is in a mess. Nor is it because they don't have problems themselves for they do. Jesus warned, "In this world you will have trouble." But immediately He added these words: "But take heart! I have overcome the world" (John 16:33). This is the key to our joy: Because of Christ's death and resurrection we know He is with us, and that someday we will go to be with Him forever.
I admit that sometimes Christians don't seem very happy, and one reason is because we are saddened by the suffering and evil we see around us. We know our world needs Christ, and we yearn for others to come to Him. We also know there is a difference between happiness and joy; happiness depends on circumstances, while joy the joy Christ gives us persists even when life is hard.
My prayer is that you will discover Christ's joy for yourself by giving your life to Him. God loves you, and Christ wants to give you His peace. Ask Him to come into your life today.