If it is healthy, it is gross. If it is unhealthy, it is delicious. That is the case of junk food and the pain of discipline. Ancient humans, in ancient nature, could rarely find fruit with sugar and meat with fat. If the smell and taste was pleasant and brain-reawrding only for them to be satisfied and strong, then the food was healthy indeed. Nowadays, in most countries, we have an abundance of food with an extended and better tasting pallete of food than the one wich nature gave us, but our instinctive brain cannot catch up with our thinking brain and still wants to grab every tasty food to not starve. Besides, children's taste buds adapt, so if a toddler doesn't like broccoli, a teen might love it. Additionally, media reinforces the idea that eating healthy is the pain of discipline, while eating junk food is exciting.