In this day and age, most people would argue that American imperialism in the late 1800s and early 1900s was a negative thing. To argue that it was actually positive, you would have to depend on two main ideas.
First, you would have to say that it was a positive for the United States. You could say that imperialism made the United States a more powerful and important nation. It also improved American morale by encouraging Americans to think well of themselves and their country. In this way, it was positive for the US.
Second, you would have to argue that it was good for the world as a whole. You could argue that American imperialism was actually good for the places that the US dominated. You could say that being freed from Spain was good for Cuba, Portugal, and the Philippines. You could also say that an increase in American power was good for the whole world. You could say that a stronger America has, ever since the time of imperialism, been a force for good in the world.
There are many arguments against this point of view, but these are the major arguments that could be made to explain why American imperialism was a positive thing.