As others have pointed out, the original question is rhetorical, so the speaker isn't really expecting a response.
However, since rhetorical questions in an argument have never stopped me from trying to get in the last word, I suppose one could respond with something like "Because you're an idiot" or "Because you're an idiot, that's why."
A slightly less natural version would be 'I dare because you're an idiot."
Of course, you could substitute whatever other reason you feel justifies your response in place of 'you're an idiot'.