Fighting anyone in winter is a bad idea. Russians thought so too, and they suffered considerably during the winter of 1812.
Napoleon's army was simply too big and moving on too little space, leading to food shortages for both the men and horses, and was caught by extreme rain in the first few weeks. We're talking about literally thousands of horses dying every day, forcing men to leave cannons and wagons behind. Men had not been instructed properly on how to coop with winter environments, and died by the bushes by easily avoidable mistakes. Napoleon made some serious tactical blunders, especially with regards to the cavalry, which came back to bite him in 1813-14.
Seriously, Napoleon thought he'd have small battle with the Russians after which Alexander would remain is loyal ally again, so he didn't really care about the possibly tens-of-thousand of men that would have died during the short campaign. He had not planned for a months-long campaign, and I'm pretty sure he was still denying that it had become that when he was marching into Moscow.
