It is established early on in John Steinbeck's novella Of Mice and Men that Lennie has a fixation with petting soft things. Throughout the novel he incessantly refers to the rabbits he will "tend" when he and George get their farm. In Chapter One he is carrying a dead mouse in his pocket. He tells George that he didn't kill it and "found it lyin' right beside the road" but George claims that Lennie "broke it pettin' it." Unfortunately, Lennie doesn't know his own strength and is usually lethal to any animal he gets his hands on. The incident with the mouse proves to foreshadow the events in Chapter Five when Lennie is seen with a dead puppy. Like the mouse, Lennie has basically killed it with affection. He tells Curley's wife,
"I was jus' playin' with him...an' he made like he's gonna bite me...an' I made like I was gonna smack him...an'...an' I done it. An' then he was dead."
When Curley's wife insists that Lennie talk to her she is unaware that she may be in danger. When the discussion turns to Lennie's fascination with petting soft things, Curley's wife allows Lennie to stroke her hair. When she begins struggling, he attempts to keep her from yelling and accidentally breaks her neck. As with the mouse and the puppy, Lennie had not intended on killing Curley's wife. His overwhelming strength simply got away from him and his powerful hands, which earlier crushed Curley's hand, easily leave the girl limp and dead on the floor of the barn.