what a sacrament of penance

Then you must travel North and south and only food that's from the mouth of a dog may you take in and speak no word of good or ill. (Line 295)

     I am still wondering why he had to take food only from the Dog's mouth to show how sorry he was. I thought about going around all the people's family he has done wrong. What

does the doggy have to do in all his sins? just leave the dog alone he too doesn't want to mouth contact with you.