Because they're your trade partners, and military allies, and potential enemies.
Or, as I like to put it, a man who keeps stealing mopeds is an obvious cycle-path.
Trade partner? Anything important that the US produces, with the exception of hollywood movies and video games? No? Last time I checked, all products I use are Austrian/China/EU made stuff. We don't need you. And we don't need military protection from you either.
Why should the US all of a sudden build consensus with the rest of the world community anyway? They did pretty well by doing what they please on their own. Like attacking Iraq... Again, I don't see why Obama will change that. With the exception of some visitations, hand shakes and shallow speeches "We're your ally", nothing will significantly change. There's no such a thing as a collaborative effort of the world community. In fact, we're all in an (economic) war. Who wins? EU vs. Russia vs. US vs. China. Who wins the battle?