What causes lead a great warrior such as Achilles to sub come to his feelings of intense anger? Three causes in Homer's Iliad was the death of his best friend, Patroklos, by Hecktor, the taking of Briseis, who was his war prize, away from him by Agamemnon, and not having the pride of a warrior to fight in battle.
First, war prizes were very valuable to warriors. This showed property or ownership of a person and how good of a warrior they were Briseis was the prize of Achilles. Agamemnon wanted to take her away from him as reparations from the war. This was the start of his wrath of anger. This showed disrespect such as the words recited by Agamemnon "But I shall take the fair-cheeked Briseis your prize, I myself going to your shelter, that you may learn well how much greater I am than you..."
(Book I, lines 180-182).
Second, Patroklos was Achilles's best friend. When Achilles's had lent him his armor, Hecktor mistakenly killed Patroklos believing he killed Achilles. After Patroklos was dead, Achilles asked for his body back so it can have a proper burial ceremony. "But what pleasure is this to me, since my dear companion has perished Patroklos, whom I loved beyond all other companions, as well as my own life. I have lost him." (Book XVIII, lines 3-5). This statement shows how much Patroklos meant to Achilles because the life of his best friend was taken and why his anger grew so strong.
Lastly, the glory of war meant everything to a warrior. It was where Achilles pride and honor originated. Since Agamemnon had insulted Achilles in such a way, Achilles knew how to hurt the Achians since he was their best warrior by refusing to fight.