There is never a way to get the best of both worlds, so they are destined to fight to the death. Therefore, Tobirama never imagines the possibility of peaceful coexistence between the two races, let alone the peaceful and beautiful scene of him and Izuna drinking and talking. Even though he knew they were wrong and painful, his heart didn't want to be controlled by him. If he had his life to do over again, Tobirama would probably still fall in love with the person who made him suffer all his life.