Answer:
They shared ethnic ties.
Step-by-step explanation:
If we track back to their history, Germany and Austria-Hungary were once part of the Holy Roman Empire and later became part of the Imperial State of Germany. They basically shared the same ancestry before their separation in 1871. Both Germany and Austria-Hungary remained close ally even after they run their government separately.
On top of that , they also shared a same vision. They wanted their ethnicity to be the dominant power in Europe. So they formed an alliance together with Italy and formed a Triple Alliance in 1882. (Italy also had a strong relation with the Roman empire)