While I was surprised (as was he), I think it reflects more on his intense efforts at diplomacy and opening up communications with countries, plus the work at reducing the nuclear arsenal.
Or, as the committee said, they praised Obama's creation of "a new climate in international politics" and said he had returned multilateral diplomacy and institutions like the U.N. to the center of the world stage.
I do believe that sometimes the award is making a political statement. When Carter won, it was considered a slap at Bush's Iraq policies. However, more often, the prize is awarded to encourage those who receive it to see the effort through, sometimes at critical moments.
Edit to Mika: Yes, someone nominated him in February. The committee had eight months to look through the over 200 nominations.