“Your words were found, and I ate them.” This statement in Jeremiah 15:16 is generally taken by scholars to be a general statement, meaning that as Jeremiah got the Word of God from various sources, including revelation, he digested them and got great joy from them. Although that is certainly true, the word “found” is matsa (#04672 מָצָא) in the niphal aspect, and it literally means “to be found; to be discovered.” Only a few years before Jeremiah started prophesying, Manasseh had reigned over Judah for 55 years, and the vast majority of that time he had been extremely evil. He repented, but the evil he had done was not reversed and set in motion disaster for Judah many years after his death in the reign of king Jehoiakim (2 Kings 24:3). After Manasseh, his son Amon ruled Judah, and he was very evil also.
During that long time of evil, the scrolls of the Old Testament were apparently lost. Jeremiah started his ministry in the 13th year of the godly king Josiah (Jeremiah 1:1). In the 18th year of Josiah, when the Temple was being refurbished after years of neglect, the scrolls of the Law were found in the Temple (2 Kings 22:8-10). It was the first time Josiah the king had seen the scrolls of the Law, as is apparent from his reaction and the reform that he then started (2 Kings 22:11-13). Since Jeremiah lived in Anathoth in Benjamin, only 5 miles or so from Jerusalem, it is almost certain that he had never seen the scrolls of the Law either. When Josiah read the scrolls, he started a kingdom-wide reform. When Jeremiah read the scrolls of the Lord, he “ate” them, and they were to him the joy and rejoicing of his heart.