Answer:
Jimmy had this feeling because his mother not scolding him because of his disobedience felt like he had suddenly grown up and was now left alone to make decisions on his own, something he was not used to.
Step-by-step explanation:
Jimmy Swanson insisted on going for the hike with Paul even when his mother did not grant him the permission. He felt caged because his parents always monitored him and made important decisions for him. Now, however, he flouted his mother's instructions, went on the hike, and even invited Paul to come over to his place.
Strangely, his mother approved of his decision as against the scolding which he expected. This was new to him, and left him feeling like he was now alone in the world to make his own decisions. It is just like seeking freedom and when you finally get it, you discover that it comes with responsibilities.