This mistress felt that she had a careless driver and she did not want his carelessness to continue. He had to change and he had to do it immediately. That is why she made him a boot slave and she forced him to lick the soles of her boots. They were dirty but he had no choice other than to lick them as clean as he could before she forgave him.
Mistress Joschi was mad at her husband and he was willing to do anything to make her forgive him. He did not want her mad at him and he wanted to have sex with her so he was prepared to do anything. She noticed that and she realized she had always wanted to try boot licking. So she got him to lick her boots and he did it to her surprise.