There is no joy like love. There is no joy but love. Christ had just spoken of the Father's love and His own abiding in it, and of His having loved us with that same love. His joy is nothing but the joy of love, of being loved and of loving. It was the joy of receiving His Father's love and abiding in it, and then the joy of passing on that love and pouring it out on sinners.
Religion is meant to be in everyday life a thing of unspeakable joy.
Because they do not believe that there is no joy like the joy of abiding in Christ and in His love, and being branches through whom He can pour out His love on a dying world.