Just Because

Arthur was rolling back and forth on the expensive persian rug, rolling as if he was bound hand and foot. He had a look of intense concentration on his face.
He rolled to the right.
Elizabeth walked past the living room where Arthur was now rolling towards the left.
She watched him for a moment.
"Why are you doing that? Arthur, why are you doing that?"
Arthur rolled to the right.
"Because it feels good," he said, biting his lip in concentration.
He rolled up to the feet of the antique coffee table, then started rolling to the left.
Elizabeth began to cry.