For an answer to this question, please check out the following link:
In Homer's Illiad, Achilles is embodied by ideas of Greek heroism. He is the representative of kleos, or the Greek virtue of glory in war. Ancient Greek heroes must always strive towards kleos--and only those with arete (unparalleled excellence) can actually attain it. Achilles is characterized again and again by his unparalleled skill and strength in war. He is the one who finally defeats Troy's best warrior, Hector. Another example of his arete is the fact that he fights the river Skamandros (that's right... a river...) and wins.
Achilles is also an exceedingly proud person. When the Greek general Agamemon takes Achilles' war prizes for his own, Achilles' hurt pride causes him to declare that he will no longer fight for the Greek side. He even asks his mother, the goddess Thetis, to ask Zeus to strengthen the Trojans while weakening the Greeks. He wants Agamemnon and the Greeks to realize how much they need him. If Achilles were a "normal" human, this might characterize him as petty. However, the Iliad takes great pains to emphasize the fact that Achilles' anger is not the anger of a regular man. The very first lines of the Iliad declare Achilles' rage as menin, a Greek word referring to the rage of gods. It is clearly defined as an emotion that only gods can feel. Thus, Achilles' anger is his true defining characteristic in the context of the epic.
This menin is very telling. When the epic first opens up in media res, it appears that Achilles' rage, which will affect the entirety of the war, comes from his arrogance and his hurt pride because he feels that Agamemnon snubs him. However, as the story progresses, events occur that alter this perception of Achilles. The turning point is when Patroclus, Achilles' beloved cousin and friend, borrows Achilles' armor and is killed in battle because of it. At this point, Achilles' rage becomes the rage of revenge and love. His capacity for such personal loyalty complicates the vision of a simple hero looking for kleos.