After the franco-prussian war and the unification of germany both nation straight up hated each other and were eager to rebuke each other in the first world war and later on the second world war. But how did it happen that this hatred just "disappeared" and is replaced with a really healthy relationship between those countries today? Sorry if this is the wrong sub to ask such a question!
The hatred did not disappeared overnight. A big influence on post WWII European politics was the creation of spheres of influence by USA and ESSR. France received a lot of American $ in aid, as did other european countries that ended up in NATO. That was also the case with West Germany. By the time Germany was unified, France and West Germany had spent decades of no active conflicts and trading, under the financial and cultural influence of the USA. Then with the creation of the European Union, it became much easier to travel, trade and live in other EU countries which also helped to bolster the idea of a peaceful Europe, especially among the newer generations of French and Germans who could easily go to each other's country and see that they didn't have that many differences or things to split.
I know that my answer is far from a historian's approach, but I still consider it to be a valid answer due to the nature of your question. By being under common influence and restrictions, then by eliminating physical barriers, the majority of the population shifted from being hateful to cooperative.