They are accustomed to a hard and impoverished existence, and therefore do not hold on to life all that strongly. The average person has less worth than a bicycle. A rapid birthrate quickly replaces any losses. They have a type of primitive toughness that one cannot call bravery. It is entirely different.
Bravery is a kind of spiritual courage.
The toughness with which the Bolshevists defended their bunkers in Sevastapol was more a bestial drive, and nothing could be more mistaken than to assume that it was the result of Bolshevist views or education. The Russians were always like that, and will likely always remain so. It is also easier to throw a life away when there is no promise to it than when, even at the moment of danger, a distant paradise still seems to beckon. One does not need to speak of the enormous danger that the armed uprising of such stolid millions is for Germany and all of Europe. For attacking soldiers, the motive of the defenders is not particularly relevant.