In "The Red Badge of Courage," what makes Henry feel like a "mental outcast"?

There is a quote in the book that says, "Some of the tall one's companions cried with emphasis that they, too, had evolved the same thing, and they congratulated themselves upon it. But there were others who said that the tall one's plan was not the true one at all. They persisted with other theories. There was a vigorous discussion.

The youth took no part in them. As he walked along in careless line he was engaged with his own eternal debate."

So Henry didn't associate or interact with anyone as they were marching. Everyone else engaged in conversations and debates, and Henry kept to himself and focused on his thoughts. This problem that Henry was dealing with greatly affected and bothered him, and he spent all of  his time focusing on it.