While you may feel that you enjoy junk food just because it tastes so good, there is a scientific explanation for why you want to have more of it. The brain naturally encourages you to seek experiences that you find pleasurable, including eating tasty food. This encouragement from the brain is known as the 'reward' system. When a person eats tasty food (including junk food) the reward circuit in the brain is switched on. This releases a brain chemical called dopamine. The chemical rush floods the brain with pleasure and so the brain creates more receptors for dopamine in response. In the same way that people with a drug or alcohol addiction require a bigger dose over time, you crave more junk food the more you eat it.