1 John 1:4 (CSB) We are writing these things so that our joy may be complete.

John's joy was made complete by writing his books. To know that what you have received from God is feeding others bring joy. My favorite example of this is the Sea of Galilee  and the Dead Sea. The dead see takes in the water flowing down the Jordan but has no way to send it out so all the sentiment, salt and other stuff gets left behind when the water evaporates. The Sea of Galilee collects all it's water from the many hills around it and it sends out all it's water down the Jordan. When I receive from God and never gave it away I became dead and unfruitful. But when I give away a part of what God gives me I became fruitful and healthy.

Proverbs 15:23 (CSB)
A person takes joy in giving an answer; and a timely word -- how good that is!

John found that His joy became full by seeing others grow closer to God.  It should excite anyone to see someone become a christian and grow because of our small part we played. I wonder if John is still receiving "credit" for his gospel winning people to Christ. We know that anytime we give to anyone who is preaching the Gospel across the globe we get "credit" for helping them.

3 John 1:4 (CSB)
I have no greater joy than this: to hear that my children are walking in truth.