Either because I'm not hungry, because I don't have any food, because I don't like any of the available food (whether for taste, health, religious, cultural or moral/ideological reasons), because I'm busy with something else at the moment, because it's otherwise (e.g., socially or physically) an inopportune time to eat, or because I'm something outside the set of things that eat (e.g., I'm an inanimate object)