Commentary Purg XI 73

Dante's bending down his face is a natural action taken in order to see his interlocutor's face, yet it, too, reveals a moral significance (see note to vv. 46-48), as the protagonist's own words will later confirm ([Purg XIII 136-138]), when he will admit that, once he returns to the mountain in the next life, his head will be lowered under the same load he now is able to observe upon the backs of others.