When he was born Achilles was foretold by seers and prophets to die young. To prevent this his mother dipped him in the river Styx holding him by the ankle. The areas of his body, were nearly all covered, but for his ankle. In combat he was considered invulnerable, many then thought that his mother's act had made him invulnerable.
But in combat, Achilles had become prideful. He took risks that some thought seemed suicidal. Or that, instead, he believed that he was immortal, a god acting among men. But the brother of his most talented adversary, hardly an apt warrior, fired an arrow that went errant and struck Achilles in the vulnerable heel and ankle. At first Achilles was shocked that anyone would deign to strike him. Then he became horrified, that he felt pain for the very first time in his life. He eventually pulled the arrow out, poison now running through his body, Achilles knew he was neither god, nor immortal.
“Achilles glared at him and answered, "Fool, prate not to me about covenants. There can be no covenants between men and lions, wolves and lambs can never be of one mind, but hate each other out and out an through. Therefore there can be no understanding between you and me, nor may there be any covenants between us, till one or other shall fall” Homer
His inability to be hurt, led him to be rash in his actions, unwilling to believe in the fragility of his own flesh.
"Fear, O Achilles, the wrath of heaven; think on your own father and have compassion on me, who am the more." Homer
“And overpowered by memory
Both men gave way to grief. Priam wept freely
For man - killing Hector, throbbing, crouching
Before Achilles' feet as Achilles wept himself,
Now for his father, now for Patroclus once again
And their sobbing rose and fell throughout the house.”
“It is always as it was between Achilles and Homer: one person has the experience, the sensation, the other describes it." Friedrich Nietzsche