All very interesting answers, but the truth is a bit more simple than what is being explained. It is not the job of the USA to be the police of the world. It never has been. However, It happened after World War Two. It was not by choice or want but was thrust upon America. Remember, prior to World War Two, America was a isolationist nation. Stability in the world was destroyed after the war. Now it was propped up by two Nations; The Soviet Union and the United States. Even during the 40 plus years of the cold war, there was stability or as much as can be achieved in the world, but then the Soviet Union imploded. It imploded because it economically and financially in ruin. This eventually happens to all totalitarian state regardless of the "Marque" they go by such as; Communism, Socialism, or Fascism. We know this because history teaches us this. Have you ever heard of the former Soviet Union. With the fall of the Soviet Union, there remained one super power that retained its economic and financial strength.
In short it was thrust upon us. Ask yourself these questions: What would happen to the world if the United States was removed from it? Would there be more or less chaos?
It is nothing to be ashamed of to be the Worlds Greatest Power. History teaches us that and so think of the Roman Empire. In the Roman world, Rome created stability and when Rome fell, the world was torn apart and hence the Dark Ages was created.
As in Roman times, the world are teetering on the strength of America. As America becomes weaker, its influence begins to diminish and so the stability of the world will become more erratic. You can see it happening now. As America's economy and finances weaken, it is creating uncertainty in the world. I will conclude with this question. What happens when uncertainty rises?