What is a reason that America should be imperialistic?
Of course, not everyone would agree that the US should act in an imperialistic manner today. There are many people who would say that we should not. If, however, we do need to give a reason why the US should be imperialistic, we can say that the US is the country that is most able and most likely to act in the best interests of the world. Therefore, if we control more of the world things will be better.
If the US were imperialistic, it would mean that we would control more of what happens in the world. We would have much more influence over what various countries do and what happens in those countries. If we had this, there would be fewer bad things happening in the world. If we controlled Syria, ISIS would not be able to have a stronghold there and Bashar Al-Assad would not be able to commit atrocities against his people. If we controlled Pakistan, it would stop committing terrorist attacks against India. America has, you can argue, better values than most other countries in the world. We care more about justice and human rights than they do. Therefore, the world might--at least in that sense--be a better place if we were in control.
The US is the country with the greatest amount of military and economic power in the world. We are the only country that could really be imperialistic in a large-scale way. We are also a country that values good things in the world. If we controlled more of the world, we would use our power to work for justice and the benefit of humanity. Therefore (you can argue) the US should be imperialistic.