Germany was in the Triple Alliance to counter it with Austria-Hungary and Italy. Germany was ambitiously trying to be a colonial power and needed to counter British control of world trade and sea lanes. With the second reich, Germany now being under one government merged by Otto von Bismark, they had the potential to be the greatest nation in Europe. A united Germany with its resources and industrial might was a contender to out shine the Britain, France, Russia.

Britain's foreign policy for centuries was always to switch sides and align themselves with whoever kept a divided Germany and divided main land continent. Germany knew this, and knew they had the potential to out grow Britain, if they could acquire overseas colonies.

Essentially they were a contender.

Keep in mind also that it was very unclear which side the US would align themselves with, in the event or War. Germany knew this. It could've just as easily been a scenario in world war one, where the US would align themselves with Germany. Millions of Italians and Irish and Germans in the US. Much hatred to British system of government. While World War I raged in Europe, a German U-boat commander in 1918 met the mayor of Baltimore and was given a parade through the streets of Baltimore, before refuelling and heading north to try and torpedo British and Canadian merchant ships.