Why did the Soviet Union desire "defensible" borders and "friendly" regimes in Eastern Europe ?

To answer this question, we must look to the Russian/Soviet experiences in the First and Second World Wars. In both conflicts, German troops (as well as those of Austria and Hungary in WWI and Romania in WWII) had invaded the homeland, causing unthinkable destruction. In World War II, for example, the Nazi invasion that began in 1941 led to an estimated 27 million Soviet deaths, almost 90 times the amount suffered by the United States in the war. The war hit home for the Soviets, in other words, in a way unimaginable in the United States. So among the primary foreign policy objectives pursued by Joseph Stalin after the war was to secure the western border of the Soviet Union. In his mind, the most effective way to do this was one consistent with communist ideology--to establish communist governments in Eastern Europe. The United States and Great Britain, of course, saw these efforts as Soviet expansionism, just as Stalin saw American spending on rebuilding in Western Europe as American imperialism. In this disagreement, born out of Stalin's obsession with creating a sort of "buffer zone" between the Soviets and the rest of Europe (admittedly, he was also not averse to spreading communism beyond Eastern Europe) lay the origins of the Cold War between the United States and Soviet Union.