The simple answer is that it was a nightmare living under the rule of either Josef Stalin or Nazi Germany, and that Belarus, like its Polish and Ukranian neighbors, was drenched in blood. Given its geographic location in the western Soviet Union, Belarussians first experienced starvation and deportation on a mass scale during the period of forced collectivization of farms. In Stalin's paranoia and self-delusion, he consolidated his power by eliminating not only all enemies, but all potential enemies, and then additional people after that to terrorize the population into subservience by fear. In Belarus, this meant more than a million people died under Stalin before the war with Germany began in 1941.
When the Germans invaded, their conquering army was followed by killing squads, "einsatzgruppen", who recruited local help and carried out mass arrests and executions of Jews, Poles, and communists. A milion Belarussians were murdered in the first year of the occupation. They had first greeted the Germans as liberators, preferring anything at that point to Stalin and his brutal NKVD. It did not take long for the Germans to lose this initial support. The German Army, SS and police forces carried out reprisals of killing civilians for the attacks of partisans. In most cases, this involved wiping out entire villages in one stroke, often in reprisals for partisan attacks that never happened. An estimated 400,000 civilians were murdered in reprisals.
300 - 400,000 more Belarussian citizens died as soldiers in the Red Army, or as prisoners of war in German labor camps. In all, almost half the population of Belarus died between 1930 and 1945.