"He must increase, but I must decrease." (John 3:30) The word αυξάνω (to increase or become more important) is a word that is used in reference to φως (light). In verses 19 -21 John has used the word light 5 times and now in verse 30 he uses αυξάνω in reference to Jesus. The light of the world is going to shine brighter and brighter from this day forward. John's light will be on the diminish and all attention will be shifted to the Lord of glory. The word ελαττοω (to decrease or become less important) is another word that has to do with φως (light). John is emphasizing the growing brightness of the Son of God and the diminishing of his own light.
This is a glorious picture of the sanctification of the believer. What a joy it should be for the Christian to have an ever growing desire to make sure that Jesus shines brightly. What a privilege it must be to take the light off of ourselves and turn the spotlight to the Lord Jesus. In Hollywood all the lights point to the stars on the main stage, but those who are truly great turn the light off of themselves in order to point all attention unto the one who deserves it. These things will be true in heaven. There will be no need for the sun by day or the moon by night, because the glory of king Jesus will illuminate the place.
Perhaps prayer would be a good time to reflect upon these truths and ask the Holy Spirit to help us become more humble and to show us ways we can turn the light upon the Lord Jesus.