Hitler felt that the German people needed more geographical space and believed that it should be had from the east.

Nazi racial theory held that the Slavs were 'untermenschen.'

The belief that a Nazi-Soviet conflict was inevitable and that if the Nazi's didn't take the offensive as soon as possible the industrial capacity of the Soviet Union would soon produce enough war materials, along with the re-organization of the Russian forces themselves following the purges of the 30's, would make them unbeatable. There was also at the time a political possibilty that a alliance between Britian, United States and the Soviet Union may materialize.