1 John 1:4

We write this to make our joy complete.

(v4, NIV)

John is writing these things so that they may have fellowship with him. In addition, he is also writing these things so that his joy may be complete. It's not just his joy, but he says "our" joy. "Our" is referring to those who hear, testify and proclaim the word of life. For these people, their joy is made complete as a result of writing these things to their readers. Making these things known to others completes their joy. They are fulfilling the work of Christ in doing these things. In addition, from hearing of these things, they may come into fellowship with God. These things bring joy to those who hear, testify and proclaim the word of life.

Joy can be ruined when the word of life is not testified. Why do you think Paul instructs us to wear the sandals of the readiness of the gospel in Ephesians? We need to be ready to tell others about Jesus. When we fail to do so, we can be filled with regret and guilt. In addition, as John mentions, we can have fellowship with one another. It is a joyful thing to welcome back one who was once lost. In the parable of the Prodigal Son that Jesus tells, the father comes running with joy to the son who has returned. He puts his favorite robe on the son and holds a feast in celebration. When one hears and accepts the word of life, he is like a son who has been lost, but now is found. It is a joyful occasion when one receives life by the grace of God our father.