The Wall Street Journal published a great article a couple weeks back, which described how humans react to food differently. The picture below describes two of the systems that our body uses when we consume foods, or are around them (you can click here for a better view of the picture). The hedonic system is a system in the brain that mediates the role that sensory pleasure (smell, site, sound) plays in eating (source). It's the system that stimulates our hunger and desire to eat. The homeostatic system is the system in our brain that kicks in when we eat to fullness, and which sends signals to our brain to help us stop eating.
Research indicated that after a large meal obese individuals reacted more hedonistically to a large, sweet, and fatty dessert, despite being full, whereas the more lean individuals didn't allow their hedonic system to take over, and instead their homeostatic system kicked into full gear and helped them overcome the desire to eat the dessert.
"When obese people see high-calorie foods, a widespread network of brain areas involved in reward, attention, emotion, memory and motor planning is activated, and all the areas talk to each other, making it hard for them to resist," says Susan Carnell, a research psychologist at the New York Obesity Research Center at Saint-Luke's-Roosevelt Hospital and Columbia University and one of the investigators.