Please give one reason why the Allies went to war with Germany and one for why they went to war with Austria-Hungary in WWI.
The Allies went to war with Germany largely because of the apparent growth of German ambition in the years leading up to the war. After Kaiser Wilhelm II took over the reins of government from Bismarck, he had Germany act more aggressively to find its "place in the sun." These actions caused alarm among other European countries who feared that the balance of power would be upset and Germany would become dominant. This generalized fear was the main reason for war with Germany.
War with Austria-Hungary is typically seen as a consequence of its alliance with Germany. However, if you need a separate reason for this, look to the content of the ultimatum from Austria-Hungary to Serbia after the killing of the Archduke. The Austrians issued an ultimatum that could clearly never be accepted by the Serbians. They clearly were trying to provoke a war rather than to find a way to keep the peace. You can argue that the Allies disapproved of this conduct and went to war with Austria-Hungary because of the aggressive and unfair nature of the ultimatum.