We love junk food because manufacturers design it to activate our brain's reward system. They create the perfect combination of salt, sugar, spices, etc. which excite our taste buds, thus activating our brain's reward system, making us feel good. Now after this, when we eat regular food, as it doesn't activate the reward system, it feels less appealing. Besides this, if we compare these foods, the quantity of healthy food looks more. But surprisingly, they both contain the same number of calories. Since healthy food also contains fiber and water, they add up to the volume, thus filling our stomach. But, as the junk food doesn't have enough water and fiber, it doesn't fill us. So, we keep eating and thus, end up consuming more calories leading to obesity, diabetes, etc.