Overconfidence definitely played a role. But so did Tsar Alexander I's unconventional strategic decision to retreat endlessly into the Russian countryside instead of giving Napoleon the pitched battle that he wanted. This led Napoleon's army to overextend itself spectacularly and hemorrhage men as it went (this is a cool graphic that shows the size of Napoleon's forces over the course of the Russian campaign).

Napoleon reached Moscow and burned it, but Alexander I still refused to surrender, and Napoleon's forces weren't going to be able to just hang out in Russia until he changed his mind, so they started to retreat. You can get a sense of the abject wretchedness of the Russian campaign by reading The Diary of a Napoleonic Foot Soldier by Jakob Walter, a German-speaking conscript in Napoleon's army. In Walter's account people started committing suicide from thirst a few days after the invasion of Russia began and by the time the army was leaving Russia Walter was so hungry that when he found a rotting cabbage he excitedly carried it with him in his pocket for a week before eating it.

So was that luck? In once sense yes, Napoleon had no reason to expect that Alexander I wouldn't behave like all of his previous opponents. In another sense its not as if it was a random event like rain or something that caused Napoleon's defeat.