Answer: territorial conflicts
Step-by-step explanation:
Admittedly, there could be several reasons. I think one of the most predominant ones is definitely regarding conflicts over land, especially, for example, World War II. Germany had grown quite vast, however, afterwards, Germany was split into four occupational zones, and eventually, it was known as two different Germany's: the East and the West. This was how it was for a number of years, hence when looking at Germany united now, the memories of the vast split and rather unpleasant conditions of the past, especially the poor economy of East Germany, may arise upon looking at the map of the now reunified Germany.