"And on this [the Lord's] day thou shalt do none other thing...that thy fasting may be perfect, or in other words, that thy joy may be full. Verily, this is fasting and prayer, or in other words, rejoicing and prayer" (D&C 59:13-14). This seems quite at odds with how fasting was portrayed in the Old Testament. However, fasting is not only a way to show humility and sorrow but it is also a way of showing devotion to God and personal mastery over the flesh. It is a way to worship God. What could be more joyful than worshiping God?
Fasting is a commandment, "Also, I give unto you a commandment that ye shall continue in prayer and fasting from this time forth" (D&C 88:76). It's not something we should take lightly. Another commandment we have received is to read the words of Isaiah [3 Ne. 23:1; see also Mormon 8:23].