In this verse, John concluded his introduction and partially explained the purpose of his letter, which was to provide mutual joy between him and his readers. In the early church, many false teachers opposed the truth of Christ. Governments and surrounding cultures also persecuted those who preached the truth. This letter was intended to address these concerns and to strengthen fellow believers in Jesus.
As John grew older, he realized that his letter would not only bring him joy but also bring spiritual joy to all those who are one in Christ. A joy of the Lord in John's heart inspired him to write of the good news of Christ in his first letter. Through his teachings, he knew that he was not only fulfilling his own joy but also filling and overflowing the inner, spiritual joy of those who read his work.
Knowing that God's Word brings joy to every person who trusts in Christ for salvation, there is, however, a greater joy to be experienced when our loved ones and those with whom we share the gospel also experience the same inner thrill. This bubbles up into a spiritual knowledge and understanding of who we are in Christ and what He has accomplished for us.

Dear Heavenly Father, it is the joy of the Lord that gives me strength. It is amazing to learn from the Gospel and the epistles of John and from the writings of all the apostles regarding the Person and work of Jesus. May Your love and joy overflow my heart. In Christ, I pray that other believers will also experience the deep, spiritual joy that we enjoy when we walk in spirit and truth and remain in fellowship with Him. 
In Jesus's name I pray. Amen.