Why does Curley attack Lennie in the bunk house ?

Curley has accused Slim of eying his wife, a charge which Slim and the others laugh off. Lennie, who is still dreaming about the rabbits, also smiles, which leads Curley to confront him aggressively. Curley punches Lennie in the face. Lennie does not immediately fight back, instead crying and calling to George for help. When Curley doesn't back off, George tells Lennie to "get 'em." Lennie catches Curley's next punch in his massive paw and crushes down on his hand. George tells Lennie to let go, but Lennie only grips harder out of fear. Curley flops like a fish. By the time Lennie finally relaxes his grip, Curley's hand has been ruined. Before Curley goes to the hospital, he agrees to pretend that he has caught his hand in a machine. Lennie is afraid that he has done something bad, but George reassures him that he hasn't as the chapter closes.