"Now Ammon seeing the Spirit of the Lord poured out according to his prayers upon the Lamanites, his brethren, who had been the cause of so much mourning among the Nephites, or among all the people of God because of their iniquities and their traditions, he fell upon his knees, and began to pour out his soul in prayer and thanksgiving to God for what he had done for his brethren; and he was also overpowered with joy; and thus they all three had sunk to the earth."
Gratitude brings joy. That reminds me of President Monson's talk "Finding Joy in the Journey." He suggests that in order to find joy in life, it's important to adapt to changes, realize what is most important, and express gratitude always. I agree with him.