Answer:
There were multiple reasons but here are some of them
Hitler for one wanted to destroy his most powerful enemy or so he thought at that time which was Stalin and he knew that by destroying Russia not only would he get that biggest empire on earth" I didn't say strongest said Biggest" but by destroying them he would end the Russian France treaty and they would not be allies but he would also have many more resources for tanks and ammunition but also he would know that when he invaded Poland they would not be able to stop him.
What Hitler wasn't counting on was Russia's cold harsh winters and their allies also had allies which were the Japanese the Japanese bombed pearl harbor which caused the US and its allies to join the war.