The best answer is, France considered Alsace and Lorraine to belong to France rather than Germany.
In the late 19th century France faced defeat in the Franco-Prussian War, as a result, Germany unified under the first King of Prussia, Kaiser Wilhelm I who annexed the French territories of Lorraine and Alsace. This expansion came with an imperialistic and nationalistic influence supported by the people of the Prussian Empire, this same sentiment was also felt across the nations of Europe. Thus, when tensions finally amassed into conflict in World War I, it was no surprise that nationalist supporters saw the reclaiming of Alsace and Lorraine as a noble French cause.
