Same reason he would have made a fantastic GL - once that boy really sets his mind to something, there is no stopping him. He acts the hero completely without effort and in accordance with the part; it's only Vimes yanking him back by the collar going, "Alright, that's enough of that," which stops him becoming a full-on Arthurian figure.
...It also occurs to me that Musabetsu Kakuto Ryu operates on much the same rules as Discworld mysticism; highly metaphorical and theoretical, often with drastic consequences, and requiring extremely careful execution to do correctly.