In Chapter 3, with Slim's "God-like eyes" and calm invitation to talk, George explains why he and Lennie go around together when most "bindle-stiffs" are loners. Comfortable with conversing with Slim, George goes on to tell him that he used to play jokes on Lennie because "he was too dumb even to know he had a joke played on him." But, because he never became angry, George says that the pranks were not as much fun, anyway. Then, after one practical joke went too far, George stopped his pranks: One day when George, Lennie, and other men were loitering around the Sacramento River, George ordered Lennie to jump into the water:
"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."
Slim remarks on the ingenuousness of Lennie, "He's a nice fella," and adds that "real smart guys" are rarely ever nice.
This scene captures the Naturalism of the novella in which men, alone and alienated, become aggressive, and the guileless fall victim to this aggression and cruelty. And, Lennie represents for Steinbeck the frustration of all men during the Depression as well as the "moral isolation and helplessness that is part of the human condition." [enotes]
In the beginning of the novel, the reader learns that George stopped playing mean jokes on Lennie. The reason why is because he told Lennie jump in a river, and Lennie did. Lennie almost drowned because he didn't know how to swim. George realized he almost killed him, and that he has to be very careful with Lennie, which is why he no longer plays jokes on him.