קל"ה, הלכות פסח, ב:א
The ten types of pulses within a person get their strength and vitality through the food that a person eats. And, because the essence of the vitality of the ten pulses correspond to the ten types of song, which relate to happiness, the rectification for eating is by eating with joy. It is written, "Go, and eat your bread joyfully." The opposite is, "In sadness she will eat," which draws forth from the concept of the blemish caused by eating from the Tree of Da'as. This relates to the ten mitzvot that one does with bread as well as the ten words in the bracha said over bread, "HaMotzi Lechem Min HaAretz." All of this relates to the ten types of song, which are the concept of joy, which is the essence of the rectification of one's eating.