Hi there! I actually covered a lot of this in this post a couple of weeks back, so I'd recommend looking through that for some basic background. I'll add a couple of additional points that are worth noting when thinking about the Franco-Prussian War and the annexation.

Prussian Motivation:

I'll note a couple of books in my source list below that are probably worth checking out, but it is easy to get side-tracked into nationalism being the sole motivating factor in the annexation and in Bismarckian politics overall. Often times Bismarck himself is portrayed as purely a German nationalist. While this is not entirely true, it is highly reductive, Bismarck could more aptly be described as a highly conservative, pro-Protestant, Prussian nationalist, who sought to unify Germany under Prussia. He was also not the only actor in the Prussian state, and often had to deal with a highly confrontational and expansionist military establishment faction among others.

However, in the wake of the Franco-Prussian War, both Bismarck and the military establishment were largely in lock-step. While the annexation of Alsace-Lorraine is often portrayed as a nationalistic action, it is really important to note that it was also intended to be a strategic one too, aimed at decimating the French economy. Alsace-Lorraine possessed a huge proportion of French mining and industrial infrastructure, which combined with the 5 billion franc indemnity imposed by the Prussians at the Treaty of Frankfurt, was intended to cripple the French Third Republic. So we really should not forget this as motivation as well, as the conflicts of the 19th century were not purely nationalistic and this context is important. This attempt to cripple France largely failed, though not by anything foreseeable at the time. The French Third Republic somehow survived its early instability, outlasted the monarchists, and proved incredibly adept at checking its own military establishment/revanchist factions (arguably checking them too well by the outbreak of WWI). In addition, the discovery and development of the Briey-Longwy mines in Northern France helped to recover French industrial capacity and economic viability.

Alsace-Lorraine itself:

I would highly recommend looking through my previous post linked above as I do not know if I will have enough time to go into the same depth here (though I may edit in some more notes later). However, I think the fairest way to put it is that there really was not a huge pro-German faction in Alsace-Lorraine during the period and a fair amount of antipathy towards the annexation following the war. For nearly twenty years the political representatives from the province were protest candidates, and protest candidates would continue to have a significant presence up until WWI. While many French speakers would leave, there were also many who remained, particularly in the cities and in trade centers (where they had always been in larger numbers).

As to why there was a certain amount of friction against German annexation, there are many reasons. Alsace-Lorraine was primarily Catholic and was now under the Protestant dominated German Empire. The annexation caused economic issues due to the emigration of thousands of French speakers. And to drive home why we shouldn't overplay nationalism in analyzing this, there was a decent divide linguistically and culturally between Alsatian (the local Alemannic dialect which was predominant among laboring classes) and what many saw as proper German. This did not result in massive unrest, at least initially, but would cause issues closer to WWI with incidents like the Saverne affair with the Prussian military in the province. However, many in Alsace-Lorraine had no major problems code-switching to interact with the French population and incorporating French into their own speech, as well as lacking any overall affinity for Prussia overall. In short, while there was no major unrest or strife against German annexation, it was certainly not looked on fondly and to a certain extent resented. This was by and large because the province had gone from a liberal and commerce-friendly Second French Empire (which they had been a part of for centuries) to the highly conservative, militaristic, and overwhelmingly Protestant German Empire. So while nationalism was certainly an element of Alsatian politics, autonomy often played a central role in the conflicts of the following decades.

I hope this helps and let me know if you have questions or want more follow up.

Sources:

Franco-Prussian War by Michael Howard

The First World War: Volume I – To Arms by Hew Strachan

Alsace to the Alsatians? Visions and Divisions of Alsatian Regionalism, 1870-1939 by Christopher Fischer (probably the best English language source on this stuff)

Iron Kingdom: The Rise and Downfall of Prussia 1600-1947 by Christopher Clark

The Franco-Prussian War: The German Conquest of France in 1870-1871 by Geoffrey Wawro