The dirty not-so-secret of the Monroe doctrine is that it rested in large part on a British naval commitment, as the Royal Navy was the only force large enough to patrol the Atlantic. During the independence struggles of the various Latin American republics, it suited Britain to keep the resources of these former colonies of European empires independent and their resources hence beyond the reach of Britain's European rivals. The immediate goal of the Americans in the Monroe doctrine - to keep the new republics independent - was the same. Still smarting from the War of 1812, the Americans resisted a joint statement or co-ordinated joint action with the British, but the doctrine would not have been credible without Britain's interest in upholding it.
One of the verities of American diplomacy, and particularly early American diplomacy, has been an opposition to any system based on the balance of power. The Monroe doctrine was aimed at preventing the export of this "system" to the Western hemisphere, meaning that power politics would not intrude into the New World and the U.S. could remain aloof from European conflicts. Allowing European countries to compete for dominance in the Western hemisphere would have risked power struggles in the New World and also obliged Americans to concern themselves with the balance of power in the Old.
With a continent laid open before them, the national energies of Americans were absorbed with manifest destiny; steering clear of the dark ways of Europe hence served practical and ideological purposes for a people whose national identity was based on a rejection of Europe. It also blurred the fact that the supposedly "isolationist" early republic was in fact engaged in an expansionist foreign policy on the frontier that would have impressed any realpolitiking prince.