For Harry, it's a chain of logic that is clear and natural. He must believe, true though his cause was, he was fairly and honestly defeated. Given that, despite his father's disloyalty, despite his own reckless ways, it must be that Hal earned his throne.

And besides-- and this, even if pressed, he would not know quite how to say-- if he will not stand for this King of England, this King the strange ways of this place has gradually brought him to respect-- then what country can he stand for? Is he still English at all?

But no one does press him for a motive, so he says to Enjolras, "Ay, I know it." And to Brienne, "'tis done, and far away, and matters not. There is none of us shall see it. --but come, what now shall we try?"