No matter how little one eats, a blessing must be said first. One must say a blessing even before a crumb. If one eats without saying a blessing, it is considered as if he had stolen the food from Hashem. All the universe belongs to Hashem; He is the Creator of all things. How can a mere mortal have the audacity to taste Hashem's food without permission? No one would dare eat a meal in another's house without being invited. In many cases, invitations must be sent out well in advance. Even a guest does not begin eating without being bid to; otherwise one is ashamed to begin eating. If one is ashamed to eat the food of another human being without permission, how can one behave so towards Hashem?
(Rabbi Yaakov Culi)