What happened that made George stop playing tricks on Lennie in Of Mice and Men?

George stopped playing tricks on the childlike Lennie after Lennie entrusted his life to George and nearly died. 

When George Milton and Lennie Small arrive at the ranch where they have been hired to work, they meet old Candy, the boss, and his son Curley. Later, they meet the skinner named Slim, who possesses great understanding because his ear "hear[s] more than [is] said to him." As they converse, Slim remarks that it is odd that...

George stopped playing tricks on the childlike Lennie after Lennie entrusted his life to George and nearly died. 

When George Milton and Lennie Small arrive at the ranch where they have been hired to work, they meet old Candy, the boss, and his son Curley. Later, they meet the skinner named Slim, who possesses great understanding because his ear "hear[s] more than [is] said to him." As they converse, Slim remarks that it is odd that George and Lennie travel together. However, George tells Slim, "It ain't so funny, him an' me goin' aroun'" with each other because he and Lennie are from the same town. George explains that Lennie's Aunt Clara took care of Lennie until she died. After his aunt's death, Lennie started to go along with George as he went "out workin'." George adds that they "got kinda used to each other after a little while."

Then, George tells Slim that he used to tease Lennie and play jokes on him because doing so made him seem very smart in comparison. He adds:

"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. Why he'd do any damn thing I tol' him....If I tol' him to walk over a cliff, over he'd go."

George took advantage of this faith one day. Since he had an audience of several men, he boasted of his power over Lennie. To demonstrate this power, he told Lennie to jump into the river below where they were standing. Lennie jumped, but only then did George realize that Lennie could not swim. Lennie nearly drowned before the men could reach him. When George finally managed to get Lennie pulled out,

"...he was so...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."

George feels responsible for nearly killing Lennie, and he no longer teases the childlike man. Also, he has become "used to goin' around with a guy," so Lennie and he are now friends.