Why did we become the world's policeman?

The ever-effervescent Jane Galt poses this obvious yet profound question:

...why did we agree to be the world/s policeman? The rest of the developed world essentially opted out of military development in favour of building their welfare states--why didn't we? After all, we were perhaps the country least threatened by the Soviet Union.

I don't think the standard imperialist answer holds. Sure, we have done some unsavoury things in order to promote our country's economic interest, but shockingly fewer such things than any other country I can think of. The US has generally pursued its imperialistic expeditions in ways that are fairly altruistic -- either ideological, or in pursuit of broadly stabilising actions such as trying to keep the Middle East fairly peaceful so that oil continues to flow, an action that benefits anpetrological countries far more than the US. Why did we take on the superpower project, and why didn't we exploit our role as much as we could have?

My take: Even if the rest of the world hates us (debatable, in my view, but let's say), being the world's policeman is high status in American eyes. On top of that, most American leaders, and most ordinary Americans, think it is the proverbial "right thing to do," and think that we do a relatively good job of it. Plus we get some economic benefits, such being able to bribe or bully people to open up their markets or buy our Treasury securities. When relative status, perceptions of right and wrong, and (some) economic self-interests all push in the same direction, the mix is potent. For the clincher, people have strong psychological tendencies to want to feel "in control." Many people fear flying so much, precisely because they feel they have no control over the risk. Forget about the world, some people even try to control their teenagers, how is that for a laugh?

Or you might pose another, simpler question. How many Presidents run for a second term? Almost all of them. It's not good for their income, and arguably it is not even good for their happiness. Most look like hell when they leave office. But they like being in control.

The bottom line? Whether you like it or not, America is not going to give up this policeman role anytime soon.