In the third chapter of Steinbeck's Of Mice and Men, Curley thinks that Lennie is laughing at him and then attacks him physically. Initially, Lennie does not defend himself because he knows that if he does, it will bring trouble for himself and his friend George. This is hinted at by Steinbeck's comment, "Lennie looked helplessly at George, and then he got up and tried to retreat."
It is not until George gives the command for Lennie to fight back that Lennie protects himself. In fact, George has to tell Lennie three times to defend himself before Lennie grips Curley's hand and stops the man from hitting him.
Ultimately, after Lennie crushes Curley's hand, it is agreed that Curley will say that the injury occurred as a result of an accident with a machine:
“I think you got your han’ caught in a machine. If you don’t tell nobody what happened, we ain’t going to. But you jus’ tell an’ try to get this guy canned and we’ll tell ever’body, an’ then will you get the laugh.”
As the quotation indicates, if it were discovered that Lennie had injured Curley, then Lennie would probably have been fired. Likewise, Curley would suffer a great deal of embarrassment if his wife and others found out that Lennie had defeated him.
In Steinback's Of Mice and Men, Lenny allows Curly to beat him up before fighting back and protecting himself for a couple of reasons.
The first being Lennie, although being mentally and emotionally challenged, does have the ability to feel and think. Lennie feels a sense of shame and guilt for his actions, knowing they were not right and accepted by society, but not having the ability to control his impulses, thus knowing he needs to be punished for his wrongdoings.
Secondly, Lennie's enormous amount of trust in Curly, enforces Lennie's understanding that if Curley decides to punish or assault him, he (Lennie) must deserve it. It is not until Lennie actually feels pain, thus causing furher confusion and need to stop the pain, that he decides enough punishment has been inflicted upon him, at a very minimal thinking level. His inability to cognitively understand actions being performed in a very stressful situation further causes confusion of why he is being punished, and way Curly is so mad and abusive to him.
I apologize, Curly is not whom Lennie trusts, it is George, please remove last post....