I'd always heard that he stole the crown from the pope to crown himself to simply say that "I'm more powerful than the Church", but I think there's more to that, because I was reading about the event and the pope kinda just went on and was cool about it. Was the reason maybe that "I was voted into this position. THE PEOPLE have more authority than the Church"?
Also, was it greed that caused Napoleon to conquer Europe or Nationalism, or what?
Napoleon wasn't a conquerer, I have written on this a few times, and fits into an Anglophone historiography that doesn't understand Napoleon as a leader.
I haven't studied the politics around his coronation enough so I can't say anything with absolute certainty. However, I will be willing to discuss anything else in respect to Napoleon's rule.