In Death in Venice, what causes Aschenbach's downfall?
In Thomas Mann's Death in Venice, Gustav von Aschenbach's downfall is the direct result of his inability to keep his sexuality repressed.
Initially, Aschenbach is able to repress his sexual thoughts for Tadzio (convincing himself that he is only attracted to him based upon aesthetics (how he looks). As he gets to know the boy, Aschenbach's feelings become harder to suppress. Eventually, Aschenbach is unable to suppress his sexual feelings for Tadzio.
By losing control of his desire for Tadzio, Aschenbach fails to consider the external issues which are arising all around him (like the cholera epidemic which is rapidly spreading). This said, Aschenbach failure to suppress his feelings for Tadzio are responsible for his downfall.