The Monroe Doctrine paved the way for imperialistic policies because it, in essence, claimed that the whole of the Americas fell within the sphere of influence of the United States. This idea led fairly naturally to the idea that the US should do what it liked, even to the point of interfering in the affairs of those other countries.
In the Monroe Doctrine, the US said that no new colonies could be taken in the Americas by European countries. It also said that any European interference with the countries of the Americas would be taken as a threat to the US. By saying things like this, the US was essentially saying that anything that happened in the Americas was the business of the US. This attitude eventually led to such things as the Roosevelt Corollary to the Monroe Doctrine and to the imperialistic actions that went along with that corollary.