The Apostle John opens the book giving the reason for writing it: that we may have full joy. What a blessing to be able to evaluate our spiritual lives against such a straightforward and easy to understand set of truths.
When your life lacks joy it may be because of fear, worldliness, disobedience or a lack of love. Turn to the book of 1st John and prayerfully ask God to reveal what could be stealing your joy.