I think this is too simplistic of an outlook. The post-Napoleonic balance of powers was not uni-polar, it was a carefully constructed and negotiated settlement by diplomats and politicians who knew the cost of war, and it lasted a remarkable 99 years. There were skirmishes in the interim, but the balance of power ensured that the bloodletting never escalated to the point of continent-spanning "world" war...until it did.

Pax Americana, by contrast, was essentially a standoff between ideological opposites that were equipped with enough nuclear weapons to assure mutual destruction. The choices were clear - coexist or die, and there were many opportunities where we narrowly escaped the second option.

You could point to many possible causes of WW1, but I think that a lot of the causes can be traced back to a hot-headed emperor who desired a larger and more prestigious empire but lacked the statecraft to do so without pissing off nearly all of his neighbors. Looking around at our world today at the number of unserious leaders who govern like a bull in a china shop, I would be lying if I didn't see any similar causes for concern.