The Immortal Master

A kind-hearted young Taoist leaves the mountain and unexpectedly starts slapping his prospective father-in-law across the face. To everyone’s surprise, instead of getting angry, the father-in-law is delighted and happily hands over his daughter to him. Meanwhile, the sweet-looking daughter laughs and says, "Well done!" What is the reason behind the father and daughter praising the little Taoist?