An important reason why Hitler believed it was necessary for Germany to go to war was the loss of territory that Germany endured after World War I.
After WWI, Germany had to follow the will of the victorious powers. These wanted to create an example of Germany so that no other country would threaten the balance of power in Europe again. Because of this, the sanctions that Germans faced were extremely harsh, and these included a great loss of territory.
Adolf Hitler believed that these harsh sanctions were one of the reasons why Germany could not fully recover from the conflict. He also believed that Europe had wanted to weaken Germany by taking the territory away, and that it was their right to reclaim the land they deserved. Therefore, one of the objectives of going to war again was to regain territory in order to strengthen Germany's position in Europe.