The Nazis and the communists were sworn enemies. The 1939 treaty was secured to give Germany the ability to challenge the West without fighting on two fronts like in WW1. They also wanted more time to rearm for a final showdown with the Soviets at some time in the future.

Interesting tidbit. The day the news broke in the German press that a non-aggression pact was signed with Russia, the NSDAP office grounds in Munich were littered with thousands of party member pins. Some NSDAP members were not happy about the pact.