William Ossiss said:
think of the worlds countries as pies. america is a fat kid. he has his fingers in all these pies. he wants them all to himself.
I would suggest picking up a history book not written by bitter communists.
During the cold war its pretty obvious why we had bases all over the world and after the cold war its because we still need to respond to threats quickly and decisively. The US isn't interested in collecting money from the local economies or taking over territory. If someone decides to threaten our nation or its people we tend to like to stomp them out before they can make good on their threats and plenty of other countries do the exact same thing.
Remember the nuclear reactor problems in Japan? Who sent the most troops in under 14 hours to help get APC's with NBC protection to help get people out of the danger zone? What country sent the most medically capable aircraft carriers to Hati to help treat the sick and wounded after the earthquake?
we also have an obligation to NATO and the UN, two organizations that demand we keep forces overseas.