The Shulchan Aruch, amidst his discussion of the laws of proper etiquette at a meal, writes (Orach Chayim 170:6) that one should not be "Kapdan" angry, nervous or uptight during a meal. The Kaf Ha'chayim (by Rabbi Yaakov Chayim Sofer, 1870-1939), commenting on this Halacha (170:29), offers two reasons why anger must be avoided during a meal. Firstly, if a person comes to a meal with an attitude of anger or anxiety, his family members will likely be reluctant to share some of the food with the needy. If a person in need comes to the house during the meal to ask for some food, the family members might feel inhibited from sharing their food with him if they see the head of the household in a state of anger. Secondly, guests and family members may not eat heartily if they see the head of the household upset. Anger and anxiety on his part causes them to feel uneasy, and they might refrain from eating out of concern not to cause him further aggravation.
The Kaf Ha'chayim adds that although the Shulchan Aruch here addresses specifically the context of a meal, in truth, one must endeavor to avoid anger in all areas of life, and not merely at mealtime. The Gemara comments in Masechet Pesachim (66; listen to audio for precise citation) that if a Torah scholar becomes angry, he loses his knowledge. And the Zohar, as cited by the Kaf Ha'chayim, comments that whereas other sins adversely affect different parts of a person's body, anger has a harmful effect upon a person's soul. The "Maggid," the angel that would appear to Rabbi Yosef Karo (author of the Shulchan Aruch) and study Torah with him, instructed him (listen to audio for precise citation) never to react angrily over anything, including matters of religion that are of sublime importance. Indeed, it is told that the Arizal (famed Kabbalist, Israel, 1534-1572) was particularly careful to avoid anger, even more so than regarding other sins.
Thus, one must ensure to avoid anger not only during mealtime, but also in all venues of life, given the particularly destructive effects of anger upon other people and upon oneself.