He didn't initially start preaching that he would be the coming leader, only that he was the drummer who was paving the way for the true leader. Eventually he convinced himself that he was that true leader - not so much through egoism or arrogance as much as being surrounded by sycophantism and a growing grassroots movement. To his credit he probably was the only one who could be that true leader he preached about.

Source: Ian Kershaw's Hitler