The answer lies in Europe's history: The continent has historically been split by long-running conflicts that pitted neighboring countries against each other.
But despite this, the nations of Europe also share much in common, as Professor John Loughlin, of the University of Cambridge's department of politics and international studies explained.
"In the 19th century there was an idea that Europe had a natural unity, that its constituent parts shared an identity, a culture, a history," he told CNN.
"People were keen to recover this 'lost unity' which dated back to before the fragmented days of the individual nation states, to previous unions, and even to the Roman Empire."
cnn