"Tom Riddle knew that I was still alive, so he must have expected that, at some stage, I would come back to resume the fight against him. And he also likes themes, such as using Hogwarts founder's items as Horcruxes. He knew that we'd been successfully hunting his other ones, destroyed most of them, even. He knew that we knew his weakness, so ... what I think ... is that in order to protect his last one, he designed its defences with the only real threat against it in mind." 