The "we were mean to them" answer is at least partly true. Among other things, it provided some ready-made grievances and problems that made militarism attractive, and it prompted the Weimar government to initiate hyperinflation to pay off the reparations.
Moreover, although the treaty was tough, it depended on the political will of (especially) the French -- which was of course non-existent.
Also, however, the Germans were not defeated in WWI the way they were in WWII. The lack of damage to the homeland allowed the militarists (who were not punished) to formulate the "stabbed in the back" excuse.
At the same time, we cannot forget the commies, who were actively trying to foment revolution in Germany, which gave traction to the various "marching societies," of which the Nazis were one.
There are a number of reasons for the hyperinflation, but this ain't one of them. Even the French and British weren't stupid enough to take inflated German marks in payment of their reparations.
It did, however, allow the government to pay off its entire debt to its own citizens for essentially nothing.