The US "good neighbor policy" towards Latin America originated with President Franklin D. Roosevelt. Roosevelt's personal beliefs about the proper relationship between the US and other countries in the Americas drove this policy.
Before FDR came to office, the US had intervened to a great degree in the domestic affairs of other countries. It had even occupied some Latin American countries to try to force them to act in ways more acceptable to the US. Roosevelt did not believe that this was an appropriate way for the US to behave. He believed, instead, that the US should base its relations with other countries on a more moral basis and treat other countries as the equals of the US. This was the origin of the good neighbor policy.