I can hear echos of Jesus’s words, as recorded in John’s Gospel. Jesus had explained to his disciples that there was an intimate relationship between he and his Father, God himself, and his disciples. He said:
On that day you will realize that I am in my Father, and you are in me, and I am in you.
John 14:20
This intimacy that the disciples experienced with Jesus, and by extension with the Father, produced incredible difficulty but at the same time, incredible joy. Joy that could not be denied.
Their joy came because they knew Jesus. Their joy came because they laughed with him, cried with him, and ultimately worshiped him. He was God who lived there on the earth with them, and they knew him and their joy increased as they were in a fellowship relationship with him and with one another as a result of Jesus.
Later, now not just writing his account of his time with Jesus, but instead in explaining the life of a believer in Christ, John says that he wants other people to know that joy that they have experienced, and by them knowing that joy, their joy, that of John and the other disciples, will be complete:
The life appeared; we have seen it and testify to it, and we proclaim to you the eternal life, which was with the Father and has appeared to us. We proclaim to you what we have seen and heard, so that you also may have fellowship with us. And our fellowship is with the Father and with his Son, Jesus Christ. We write this to make our joy complete.
1 John 1:2-4
There is a type of selfish desire that is of God. In this case, John says that he is writing what he has seen and heard in Jesus so that their joy would be complete. John says that he has written so that they, the disciples who have seen and experienced this life and light, that of Jesus, would have a complete joy. This is his desire, that the others would know Jesus and would know the Father, and in this way their joy would be complete.
They would have complete joy because they would be together with others. In a similar way that Jesus said that he is in the Father and Father is in Jesus, and Jesus is in the disciples, now the disciples desire that others would have the same understanding and that Jesus would be in them, and by doing so, the disciples would experience immense joy.
The body of Christ would be built up.
The bride of Christ would become that which the bridegroom, Jesus himself, was wanting.
And in this way, the joy of the disciples would be complete.