The Peace prize is different, by nature it is always non-consensual, because there would be no reason to award a Peace Prize if there were not someone else who is waging a war, denying other people their basic rights or doing other Bad Things. Today it's the Taliban and the child exploiters who are the bad guys pointed out by the prize. Last year it was the Syrian dictatorship, in 2011 the Chinese government.
When Obama was given the Peace Prize, I was also shocked in disbelief, and like most people I think it was not deserved. But then I saw that some people in the US were not simply shacking their head, they were foaming with rage. Coincidentally, they were the same people who supported, and sometimes continued to justify the disastrous foreign policy of the previous administration. And that make me think this prize was actually well awarded. It was not deserved by Obama, and I think the Nobel committee, by giving him the prize at the beginning of his presidency before he could actually do anything for or against peace, was almost explicitly saying that they didn't care that he didn't deserved it. This prize was a middle finger to the Bush administration and I think that middle finger was well deserved.
The same goes for other polemical awards. Al Gore got it, and it made all the climate science deniers mad. Yasser Arafat got it and the Israeli political right got crazy.
It's not always like this (The Red Cross got the prize a number of times, and nobody disagrees with it), but often, if you don't understand why a Nobel Peace Prize is awarded, look at who it pisses off.