John knows the reality of eternal life! He's "confessing" what he knows (just as Jesus did...) so that his joy may be complete

John 14: 11“These things I have spoken to you, that My joy may remain in you, and that your joy may be full. (Our joy is in relationship to his joy.)

John 16: 22Therefore you now have sorrow; but I will see you again and your heart will rejoice, and your joy no one will take from you.

Fellowship with Him = Walking in the Light = Practicing the Truth = Fellowship with One Another = Cleansed of All Sin = Confess our Sins

To know the reality of all these things is to know the joy of the Lord... and this joy must be passed on so that it may become complete

1 John 1:1-5, 1 John 1:6, 1 John 1:7-8, 1 John 1:9 and 1 John 1:10