In the course of a series of statements made by Rav Huna that are quoted by Rav Yitzhak bar Hananya, we learn that a waiter is expected to serve food without eating, except in the case of wine and meat, whose enticing aroma would make it difficult for him to do his job if he does not get to eat.
A person who is in a situation where he sees food – and certainly if he smells enticing food – but cannot partake from it, may be affected psychologically or even physiologically. This can be seen when the person begins salivating, secretes digestive acids in his stomach, etc. This sensation can make a person uncomfortable, and, on occasion (for example, when someone suffers from a stomach ulcer) lead to potentially dangerous situations. It would not be uncommon for a person who has an ulcer to suffer serious stomach pains that will be apparent on his face, and may lead to him fainting.
This is shown to be true by the Gemara that tells of Ameimar, Mar Zutra and Rav Ashi who were sitting together in front of the home of King Izgur. As food for the king was brought past them, Rav Ashi notice Mar Zutra turning pale. He quickly stuck his finger into the bowl of food, and placed it in Mar Zutra’s mouth to satisfy his craving. In response to the waiter’s complaint that he had ruined the king’s meal, Rav Ashi insisted that there was something wrong with the food, and that his actions had saved the king from eating tainted food. Eventually they discovered that Rav Ashi was correct, saving him from punishment, even as he had saved Mar Zutra from a difficult situation.