The feelings of hunger and satiety are the direct result of hormones and cell-signalling. There are four major hormones involved appetite: insulin, leptin, ghrelin, and PYY.
Insulin suppresses appetite, and is released by the pancreas when the blood sugar reaches a certain threshold. Leptin also suppresses appetite, but is released by fat tissue. The burning of fat cells lowers leptin levels in the body, increasing appetite. From this it would seem that among the obese, there is very little appetite, which is often not the case. One possible reason for appetite to be retained even in the presence of more fat tissue than average is that with larger and larger concentrations of leptin, cells experience what is known as desensitization, and become less influenced by it. Ghrelin is sometimes known as the hunger hormone, and increases the appetite. Its release is triggered when the stomach is empty, and is stopped when the stomach is stretched. PYY stands for peptide YY, and is a short peptide that operates by counteracting the effects of ghrelin. It is secreted by the small intestine after a meal.
Cell signalling plays an important role in controlling appetite. The presence or absence of certain hormone directly influences whether someone is hungry or full. Without these hormones to limit the amount of food we eat, we would simply eat until we ruptured our stomachs.