1 John , make our joy complete

1 John 1:1-4
Proclaim concerning the Word of Life, proclaim to you the eternal life.
That which was from the beginning which we have heard, which we have seen, which we have touched.
So that you also may have fellowship with us. And our fellowship is with the Father and with hi Son, Jesus Christ. We write this to make our joy complete.

The noticeably authoritative tone of 1 John and the author's claim in the opening verses to be an eye-witness support this.
Both the Gospel and 1 John belong to the close of the 1st century The Christian faith was 50 or 60 years old by this time and had spread throughout the Roman Empire. John, by tradition living out his lat years at Ephesus in modern Turkey- a strategic center of the Christian church- was probably the only apostle still alive.
Why?
Make out joy complete: No fear, no wondering, all these Word are eternal life and it is truth. John is a witness man.
Christian need to fellowship among each other and may fellowship others let them  know the spirit of Christ.
1 John was written to counter some early form of Gnorsticism' being propounded by people who were once church members but had now withdrawn from the group. They liked to think of themselves as intellectuals - possessing a superior knowledge of God. Contrary to the traditional Jewish and Christian holistic view of the world, they drew a line between the spiritual and the material. In practice this often led to immorality because nothing the body did could tarnish the purity of the spirit. It also led to a denial of Christ's human nature, which was either 'make-believe' or only temporary. They reasoned that Christ, being spirit could not have died.
In this strong but tender appeal to his 'little children' in the faith. John makes it plain that these ideas cut out the heart of the Christian message. If Christ did not become truly human and die to win forgiveness for the sins of the world, there is no Christian faith.
The word is both a person and a message: the two are inseparable. John has seen God's message of eternal life in Christ, had heard it from him, confidence born of certainty.
These message are completed with joy to all people who believes in Him.
How?
To be a Christian is filling with joy. What I fear or worry about anything? Be faithful to the Christ. Enjoy the fellowship with Christ and God.