Harry died for the greater good. When he realized that part of Voldemort existed within him, he had to risk his own life to eradicate it.
The hero must vanquish evil his own evil before conquering capital-E Evil! He must come to understand that the worst virus of all lives within himself.
Once the hero sacrifices his life for all others–Others–he will return willingly (if not, at first, reluctantly) . He returns to save the world and defeat the enemy of the world~