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George told Lennie to jump into the river even though he knew that Lennie could not swim. Lennie does jump but nearly drowns. Lennie is so grateful for being saved that forgets that George had told him to jump in the first place. Consequently, George decided to stop playing jokes on Lennie. We get the sense that Lennie means more to George than someone to play pranks on: he feels guilty for causing George pain.
George played jokes on Lennie because it made him feel smart and stronger than him. He stopped playing jokes on Lennie because he realized it was jeprodizing their friendship and he felt guilty for that.
Of Mice and Men