Malbim (15,16): There are three answers you can give (and none are proper). (a) You did not know that he is crazy - see, he is crazy! (b) You brought him like one brings a lunatic for jesting - am I lacking lunatics?! (Refer to 21:16:1:1.) (c) You did not bring him; he came by himself - if he would come by himself, is it proper to let him come?!