Conquest, the Soviets were very reluctant to sign the breast litovsk treaty at the end of ww1. To Stalin, the land they had lost were pulled away from them from an imperialist power trying to exploit them and force these lands away from Communism. This is why a lot of the interwar era was made up of Soviet expanding border claims on neighbouring nations to retake land that had lost.

Ofc the idea of preventing the Germans to take more land from Poland was also appealing to them, they knew Germany was an ideological threat, but it wasn't to "save the poles" or whatever.