Short answer: the soviets didn’t see Poland as an independent country.

As far as they were concerned, it was a part of Russia that had been occupied by the Germans. Also, controlling Poland would mean that they would be able to intervene in Germany, at the time going through communist uprisings.

Poland also acted aggressively towards the Soviets in order to secure its borders. Making war inevitable.

Another answer is: fking soviets wanted to spread fking communism to the Europe. And Poland was the first country on their way.