In Chapter 1, George spends quite a bit of time ranting and making sarcastic comments about Lennie's behavior, the fact they have to walk so far, and their lives in general. However, these rants seem to have little effect on Lennie.
Why is this?
Lennie has obviously heard it all before and knows that George will just as quickly calm down as he becomes angry.
Lennie knows that he could easily defeat George in a fight so he has no need to fear for his safety when George is upset.
In addition to being intellectually disabled, Lennie is physically disabled and cannot hear well. He does not even hear what George says most of the time.
Lennie's intellectual disability keeps him from recognizing George's sarcasm.