This is a very juicy question that somebody could write an entire PhD thesis about, so I'll try to be brief.
There are dozens of reasons that the US became and imperialist nation. As former colonies of the British Empire, the model of government was certainly familiar in our historical memory. However, America didn't instantly emerge as an imperial force that could compete with European empires. As the US gained wealth and military might, Manifest Destiny and the Doctrine of Discovery demanded they acquire more and more land. The feeling of land-hunger is endemic to empires, so this cultural norm primed Americans to support imperial ventures in foreign lands once American soil ran out.
Another reason America was primed to promote imperialism can be understood through the self-perception of being a "shining city on a hill." Though popularized by President Reagan, it dates back to the Puritans of Colonial America. To paraphrase a complicated concept, America stands out as a morally, ethically, economically, and governmentally superior nation. This nationalistic concept morphed over time, and instead of the best, we became the boss (and also still the best). At a certain point, there is a level of fear that goes with living on a fortified hill. It follows that there would be hordes of barbarians rushing the gates from the nether worlds from which they came, ready to sully the American experiment with their poisonous ways. In order to stem this fear, America repeatedly lashed out at countries they perceived as threatening the American way of life
My studies have led me to conclude one of the single biggest drivers of American Imperialism was and still is the dollar. Nearly every single imperial venture in Latin America was propelled by monetary interests of both corporations and the US government. The Banana Wars are a prime example of the US Marines being sent in to quash organic social, political, or economic reforms that could have posed a risk to corporate or federal bottom lines. Nicaragua, Panama, Cuba, Puerto Rico, Haiti and Mexico all had their shores invaded, elections stolen, and governments controlled because they were conducting their affairs in ways that the US disagreed with. Incessant American involvement in Central American affairs cut paths of destruction and destabilization that are still being felt today. Again, this is impossible to completely answer in a succinct way, so there are multitudes of concepts I'm leaving out.