How does the author try to explain this ‘destructive stage’?

In order to approach the problem of anxiety in play, let us consider the activity of building and destroying a tower. Many a mother thinks that her son is in a ‘destructive stage’ or even has a ‘destructive personality’ because after building a big, big tower, the boy cannot follow her advice to leave the tower for Daddy to see, but instead must kick it and make it collapse. The almost manic pleasure with which children watch the collapse in a second of the product of long play-labor has puzzles many, especially since the child does not appreciate it at all if his tower falls by accident or by a helpful uncle’s hand. He, the builder must destroy it himself. This game, I should think arises from the not so distant experience of sudden falls at the very time when standing upright on wobbly legs afforded a new and fascinating perspective on existence. The child who consequently learns to make a tower ‘stand up’ enjoys causing the same tower to waver and collapse; in addition to the active mastery over a previous passive event, it makes one feel stronger to know that there is somebody weaker-and towers, unlike little sister, can’t cry and call ‘mummy’