Entire country adores you, thousands of young soldiers sees you as walking Godlike figure and are willing to charge into their deaths for you.
Imagine what something like that does to your ego
He was simply addicted to war, after his first defeat in 1814 and exile to tiny island of Elba he still decided to return and try again despite all the Great Powers and rest of Europe standing united against him and France itself being totally exhausted after almost 25 years of endless war
Interesting fact, main Allied countries (Britain, Austria, Russia, Prussia) didn't declared war to France in 1815, they proclaimed Napoleon outlaw and declared war directly to him.