He has this dire obsession to humiliate Austria, and by stalking him it's easier to get blackmail on him or something. Why would Prussia want to see Austria humiliated? Probably because, historically, Prussia never really liked Austria since it was a Germanic country but had a great lot of citizens from other non-Germanic countries, so Prussia kind of saw Austria as "diluted", and therefore didn't like him. But Austria kept wanting to unite with Germany as the German Empire, which Prussia wanted too, but they couldn't join Germany together. So it created a bit of a rivalry.