I think a big part of it was a growing understanding that nobles were not the be all and end all of life. With more and more books available to even the common people, more people learning to read and gain an education, the early beginnings of the industrial revolution supplying jobs, all conspired to break the nobles' hold on the minds of their subjects and allow those subjects to believe they really could be free.
I agree, it's going to be a long road before there can be any hope of real unity, but the seeds have been planted and hopefully they will take root and grow. The best thing that can happen to mankind at this time would be for people to realize that centuries-old feuds between now mostly defunct noble houses are no longer relevant and should be forgotten.I think Europe's problems at the moment is that they are not states as the American states are/were but countries that are far more different and older and stubborn. Most of whom have ruled over most of the others at some point or other. The UKs alliance with France, for example, is very recent and we have been at war with them far longer than we have ever been at peace. So we'll never get that unity, at least not for a long while.
I won't hold my breath, though.