Gathering excessive sugar and sodium is our body’s primal need. When food used to be scarce and we were living as scavengers, whenever our ancestors came across glucose and salt, they consumed as much as possible because these are sources of energy. Hence, when you eat junk food, it sends signals of positivity to your brain even when these junk foods are harming your body. Therefore, you feel happy when you are eating junk food. It is also social conditioning. Over the years, youngsters have come to believe that when they are consuming junk food, they are supposed to be happy. Hence, unconsciously, they train their brains into believing that eating junk food is something positive.