Final answer:
The developmental change in childhood conceptions of friendship is due to cognitive development, where expanding cognitive skills allow children to understand complex social concepts.
Step-by-step explanation:
During middle childhood, children's understanding of friendships becomes more sophisticated, and they begin to link the concept of intimacy to their conceptions of friendship due to cognitive development. Cognitive skills continue to expand, allowing children to grasp more complex social concepts such as past, present, and future, which are essential in understanding relationships and the theory-of-mind (TOM). This advancement enhances their ability to comprehend different perspectives and feelings, contributing to a deeper understanding of intimacy in friendships.