Josiah was 8 years old when he began to reign. Unlike Joash, he didn't have one single counsellor that he relied upon solely for his life and decisionmaking. Still he successfully served the Lord from the outset of his reign. The Bible even goes so far as to say that he "declined neither to the right hand, nor to the left."
It all starts at the age of 16. This is when the Bible explains that Josiah truly began to seek the Lord and comes closer to Him. After he has developed a personal relationship with the Lord, Josiah turned his attention to the sin of the people. At the age of 20, he went around Judah to crumble all the idols and altars for worship of false gods into dust.
Finally, at age 26 with a pretty stellar resume of service to the Lord already established, he ordered that the Lord's house be cleaned up and brought to a state of usefulness once more. In the process, the Law of God is found.
When Josiah heard the words of the Law, he had a most emotional response. Let's listen in on II Chronicles 34:18-21,
"Then Shaphan the scribe told the king, saying, Hilkiah the priest hath given me a book. And Shaphan read it before the king. And it came to pass, when the king had heard the words of the law, that he rent his clothes. And the king commanded Hilkiah, and Ahikam the son of Shaphan, and Abdon the son of Micah, and Shaphan the scribe, and Asaiah a servant of the king's, saying, Go, enquire of the Lord for me, and for them that are left in Israel and in Judah, concerning the words of the book that is found: for great is the wrath of the Lord that is poured out upon us, because our fathers have not kept the word of the Lord, to do after all that is written in this book."
Even with the awe-inspiring works that Josiah had done in his life, when he read the book of the Law and realized that he and the people were simply falling short of the standard God had put in their lives, he rent his clothes in anger. His sin and the sin of the people put a heavy burden on his heart, that he couldn't just ignore.
So how does our sin weigh on us? Do we examine our sin as if it's no big deal because God has already forgiven it? Do we claim that because of all the other good we have done our sin in x area is meaningless? I certainly hope not!
Josiah had done much for the Lord through his young years. But that didn't change his reaction when he learned some of his actions were actually sinful. Despite all the good Josiah had done, he looked down on himself and was angry at his own sin. That also should be our response.
We can't just treat sin as if it's no big deal. When we do, it is allowed to grow and fester. Our sin needs to bring us to our knees to confess and ask the Lord to forgive.