I don't have a biological one, but I have a psychological one. It's a matter of conditioned response. Most parents try to make sure their children eat healthy foods, and reward this with candy. For example, they get ice cream if they eat all their dinner. Well, when the parents make it into something special, children automatically associate sweets with the extra attention that they receive for having done the good deed that goes along with being rewarded. They crave, and love, candy because it represents what they normally aren't allowed to have. Plus, it is also associated with special events, which is a cultural thing...for example, Halloween, birthdays, Christmas stockings, etc., all involve candy, so therefore, children are seeking to recreate the special occasion by imbibing in candy and sweets. If a parent were to reward their children with fruit, these particular children would go crazy over the fruit, whereas the others are crazy over the sugar.