From Jackdaw's link:
Vygotsky's main work was in developmental psychology, and he proposed a theory of the development of "higher psychological functions" that saw human psychological development as emerging through interpersonal connections and actions with the social environment. During the earlier mechanistic and reductionist "instrumental psychology" period of his career (1920s), he argued that human psychological development was mediated by signs that he viewed as psychological equivalent of instrument use in human labor and industry. Later, in the "holistic" period of his career (first half of 1930s), Vygotsky was increasingly influenced by the systemic thinking of the scholars associated with German-American Gestalt psychology movement. It was during this period that he—under the influence of Kurt Lewin's "Topological (and vector) psychology"—introduced the vague notion of the "zone of proximal development" and identified play of young children as their "leading activity", that he understood as the main source of the preschoolers' development in terms of emotional, volitional and cognitive development.
Is that anything to do with the need to get on with each other, grow up, knock off each other's rough edges? More difficult for single kids (like me) than those brought up with siblings.
I eventually grew up, though bypassing completely the holistic stage, and managing to avoid, more by luck than design, the vector psychology bit.