The child is impressed by his own weakness in comparison with older people, and he wishes to become their equal. I remember my boy's profound delight when he realized that he would one day be a man and that I had once been a child ; one could see effort being stimulated by the realization that success was possible.
Source: On Education, especially in early childhood, 1926, Pt. 2: Education of character, chap. 5: Play and fancy
More info.:http://russell-j.com/beginner/OE05-020.HTM