There are several reasons that people faint including lack of food, low blood pressure, hypoglycemia, growth spurts, physical exercise in excess and lack of sleep. Most of those reasons sound a little far-fetched as far as I'm concerned which is why I have my own little theory about fainting. I believe most people faint after they see the "Mayonnaise Monster." The Mayonnaise Monster smells worse than anything you can possibly imagine and he repeatedly says "let me see your bologna sandwich" until you pass out.