"There's no way for the people who produce these things to account for every possible variation of how people work their way through things, so they need to go with the version typically taught. As 'but why?' as it's likely to be, it'll at least give you a look at how people who learned entirely through this sort of thing have learned to think and get to the same conclusions you do."

Not that he's at all certain that any of what he just said will move the kid to be less of a headache about his schoolwork.