What happened that made George stop playing tricks on Lennie ?

Lennie almost drown as a result of George's jokes.

"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."