In Chapter 3, what caused George to stop playing mean-spirited jokes on Lennie ?

From the text:

"Funny," said George. "I used to have a hell of a lot of fun with 'im. Used to play jokes on 'im 'cause he was too dumb to take care of 'imself. But he was too dumb even to know he had a joke played on him. I had fun. Made me seem God damn smart alongside of him. Why he'd do any damn thing I tol' him. If I tol' him to walk over a cliff, over he'd go. That wasn't so damn much fun after a while. He never got mad about it, neither. I've beat the hell outa him, and he coulda bust every bone in my body jus' with his han's, but he never lifted a finger against me." George's voice was taking on the tone of confession. "Tell you what made me stop that. One day a bunch of guys was standin' around up on the Sacramento River. I was feelin' pretty smart. I turns to Lennie and says, 'Jump in.' An' he jumps. Couldn't swim a stroke. He damn near drowned before we could get him. An' he was so damn nice to me for pullin' him out. Clean forgot I
told him to jump in. Well, I ain't done nothing like that no more."