Researchers have found that junk food stimulates neurons in our brains that cause a sense of reward. The fats and carbohydrates (sugars) in junk foods are the reason for that feeling. When they are combined in the same food, they have a bigger effect on the brain than each would on its own. Many junk foods have that type of composition, making them especially appealing.
When mice are given either carbohydrates or given fats to consume, they can stay slim. But when mice get to eat them together, they end up gaining weight. People will also tend to select foods that contain both.
It has been suggested that happens because fats act on one reward pathway in the brain, and sugars act on a separate one. The combined effect of stimulating both at once is tough to resist.