Why would prussia seek a war with France?

Prussia was a small but increasingly important and ever larger power in Europe from the 1700's onwards, the latest in a string of German mid-range powers.

Prussia fought various wars against and alongside France over time - Prussia was establishing itself as an alternative to Austria as the main power in the Holy Roman Empire (Germany) - After getting a bit of a kicking from Napoleon in the early 1800's Prussia sent troops with the Grand Army into Russia in 1812 but switched sides again after the destruction of that army in the winter, coming to the aid of Wellington's British army at Waterloo and helping finally defeat Napoleon.

After this Prussia finally knocked out Austria and continued to gobble up the small states of the HRE. By the late 1860's having crushed the Austrians and south German states in 1866, Prussia was finally the major power in Germany and began assembling a new state "The German Empire"

France was a major threat to the new Germany and so Prussia began to seek a fight to remove them - France was still viewed as the major land power in Europe and wasn't shy of another round against whoever the dominant German power of the day was.

This leads to the Franco-Prussian war of 1870 - Prussian (German) victory leading to the total destruction of the French armies and collapse of the French Imperial Government, making Germany the new Top European Land Power, this in turn fueled French anti-German feeling and leads to the German Naval Arms race with the British since a major land power ought to have an empire and that required a fleet.... which leads to WW1 which continues the French/German feud and the settlement of which helps lead to WW2 which leads to the Cold War and the EU which has at it's heart the simple idea of stopping the French and Germans having a massive war every Fifty years or so forever.