Achilles was the son of Thetis, a goddess, and Peleus, a mortal. Thetis wanted to make him immortal by dipping him in the River Styx, but since she held him by the heel, that part proved to be vulnerable to death.
Psychologically, we can say that Achilles, so loved and protected by his mother, turned into a mother boy. A mother boy could turn out effeminate or, as was the case of Achilles, could grow with a sense of invincibility. In Achilles´ case, he goes thru life thinking that he can get everything he sets his mind to. But he is not perfect as he thinks he is. He has an Achilles´ hell. No human being can be perfect.
One of the problems these types face is a difficulty to accept commands from a superior. When Agamemnon takes from him Briseis, his concubine, Achilles gets angry and refuses to continue his involvement in the war, a painful loss to the Greek army. Only after his friend Patroclus is slain in combat, Achilles returns to the battle to avenge his death.