"The Vinegar Tasters" Response. Why is one of them smiling?

In the novel, The Tae of Pooh, we are introduced to a picture of, The Vinegar Tasters. We are told there are three masters that are sampling vinegar, and each represents the Essence of Life. The third man, Lao-tse, is smiling, whereas the other two were either making a sour or bitter face. From knowing this, we can tell Lao-tse, of the three men, is the optimist one, because though vinegar does not have the "most pleasant taste", he does not look at it with an "unappreciative mind" as the others do. Instead, he smiles. Using Hoff's response, he is smiling because his understanding changes…