Energy balance has been explained in the simples terms of calories in vs. calories out, but its not that simple. There are still additional factors that affect what food is eaten, when its eaten and how its digested. This makes the Energy In part a bit more complex in addition to counting calories. Appetite, environmental factors, psychological factors, individual variation, and food processing will all impact Energy In differently for every individual.
Gastric juices, enzymes and other hormones play a role in the desire to eat along with social factors. Physical activity, and a lack thereof, also plays a role in appetite. Physical activity too close to eating a meal can be an appetite stimulant and, for others, a suppressant. Listen to you body.
Hormones perform on specific centers in the brain that affect sensations of hunger and fullness. Its important to note that some individuals have genetic abnormalities that lead to the disruption in the cells being targeted by specific hormones. If a hormone can't function properly, the signal to stop eating may be diminished, leading to excessive weight gain. The control of appetite is closely regulated by specific hormones including leptin and ghrelin:
...is a hormone produced by ones fat cells that decreases appetite when released. It signals to the brain that enough food has been consumed. It tells the brain that we have reached a desired caloric intake.
Low leptin levels in induce appetite and suppress energy expenditure. Larger bodies (those with higher BMI's) will have larger quantities of leptin which may indicate that leptin resistance plays some role in obesity.