Another reason was political...namely the long bitter rivalry between the Greek city states and the Persian Empire as a percieved semi-civillized greek region Alexander used the "Persian Threat" to cement the Greek states into a tempory alliance based on the tried and tested means of revenge and plunder.
Ofcourse given the success of Alexander no one was complaining about the "barbarous macedonians" once the spoils caravans arrived in Greece but one has to wonder would Alexander have become Great if his campaign recruitment among the Greeks had been...
Soldiers of Greece, I ask you forget the East, where the Persian threaten to invade again and forgive the persians, the burning of our cities, the enslavement of greek wives and daughters, put out of your mind the emmense wealth of fabled Troy and instead follow me westward where Rome, a village by greek standards is fighting it's neighbors for control of central italy where resources are fairly scarce and the march hard given the mountainous terrain but if successful pottery and grain is your spoils of war....
Anyone...come on someone must want to head West...anyone at all...