I guess it is kind of silly to award the Nobel Peace Prize to a body as large as the EU, but in some ways it makes sense. Europe has just had 66 years of peace, after a gruesome first half of the 20th century, and a history of on-and-off brutal war throughout the centuries that preceded it. The ties formed by the EU are designed to ensure that wars killing millions and millions of people never happen there again. Since the end of the Cold War, the EU has helped Europe absorb lots of former Communist nations and keep those countries on a path to liberal democracy. It's a good thing.
It probably would've been better for the Nobel people to specifically honor some of the people who set up the EU -- I don't know enough of the history to know who was important. But the general idea is a good one.