The apostle John wrote three letters to Christians. Only a few sentences into his first letter, he tells those to whom he is writing that the purpose of his writing is so that his joy may be complete. This seems a little strange, and different from Paul's letters, which always recognize Paul as the author in the opening paragraph. However, there is something that we can learn from this simple statement.
John's first letter is full of encouragement and teaching for the Christians to whom he was writing. When we think of this passage of Scripture in this context, it is clear that there is a joy that comes from teaching. I suspect that we can learn something more from a couple of simple observations.
Have you ever watched a child grow up? I do not have any children myself, but I used to teach children's Sunday School. Now, when I look at those that I used to teach, I can't help but be happy about where many of them are. In fact, some of the people I have come in contact with over the years are now leaders in the church and proudly proclaiming the Gospel of Christ.
I suspect that the joy that John got from teaching the Christians in his day is very similar. He may have come into contact with some who were becoming mature in their spiritual walk. Watching them grow spiritually, and knowing that God had allowed him to have a part in that, must have made John very happy, as well. I suspect that this is what he meant when he said that he was writing so that his "joy may be complete."
What can we take away from this? Do not be afraid to teach, and not just teaching specifically. Do not be afraid to be a leader. There may be difficult times. However, we soon find that the outcome of being a leader is worth the effort put in. There may just be some more people worshipping God in Heaven, as well.