Why does the USSR want to take over Eastern Europe?

I been studying the Cold War as part of my A Level Exams but when I research outside textbooks and schools. It never mentioned why the USSR wanted to take over Eastern Europe. I know that they have the intention and how but I never knew the why.

Textbooks says that in schools of historiography, for Orthodox, it is because of Russia and communism expansionism while in Revisionist, it is for defensive purposes. Not even Quora have the answer I want. So can you guys answer for me.