Make sure your reply is informative to kid-sib. If others, without the inclination survive and reproduce less well, then the next generation will contain more of those who have the genes that cause this inclination. A special fondness for sugar, especially in children, in an environment where sugar is rare, would lead to better survival because of the energy the sugar gives to flee from predators. It was this survival advantage of high sugar consumption in an environment where sugar was rare that was the distal cause of sugar-preference, both then and now.
Cognition works similarly.