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“...and Germany’s deep sense of grievance over losing the Great War (WW I).”

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That was probably the biggest impetus, and how humiliating and stiffling the treaty of versailles was (mostly justifiable) after WWi.

But we also forget how fluid used to be before the 20th century, because we have seen the maps of Europe change so little in the last 100 years, relatively speaking. Even when the soviets took over much of eastern europe after wwii, the soviet block national borders still existed.

I believe both Germany (holy roman empire) and Russia (russian empire) not only dreamed of (re)conquering parts of europe, and they not only felt like they were owed the territory, but also that the concept of a crystalized europe was not true then. Just look at how much change there often was in borders before the 20th century. So many kingdoms and duchys and changing borders even where nation states existed.
I am in no way excusing it, but the territorial aggression and transition was the norm in Europe.

I believe both Germany (holy roman empire) and Russia (russian empire) not only dreamed of (re)conquering parts of europe,

Don't leave out the Austro-Hungarian Empire. The difference being that the Habsburgs eventually succeeded. The current EU was a construct of Otto von Habsburg who began work on the concept in the 1930s before World War II.

The Treaty of Versailles was punitive, but it was less burdensome than the treaty that Germany imposed on France after the Franco-Prussian War. As it happened, to meet the terms of that treaty, the French buckled down to common purpose and reformed their government and economy. In consequence, France paid off a heavy gold indemnity early and entered into an era of prosperity and accomplishment that became known as La Belle Epoch. Revanchism soon faded.

You are correct of course about the changeable nature of Europe's boundaries before WW I. The catastrophic nature of that war stemmed from advances in military technology, organization, and resources. In effect, Europe's armies had suddenly become more powerful and destructive. Put to the test, Europe's bumbling leaders failed to make peace even though their countries were suffering catastrophic losses.