"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 "…

…say, I agree with Hoff's response, therefore, I do not have my own individual view to this question. I could say that, "perhaps he just likes the taste" or, "perhaps he just finds the situation funny", however, because it is an "allegorical" scroll, the meaning is perhaps one which is deep. Then again, because allegorical can mean to have a timeless meaning - then there is no one meaning as to why Lao-tse is smiling.