Rich countries, with populations that are aging faster than ours, have a strong motive to save to provide for an impending sharp increase in the number of retirees relative to the number of workers. Moreover, many advanced economies outside the United States also have a paucity of domestic investment opportunities relative to the United States.
As a consequence of high desired saving and low prospective returns to domestic investment, the mature industrial economies outside of the United States, as a group, seek to lend abroad.