There are a variety of settings in the novel and Lennie reacts differently in each. He is more relaxed as he and George spend the night in the brush before reaching the ranch. Lennie is often described in the novel using animal imagery and he seems more at home in the natural environment. He tells George when they argue that he could go off and ‘live in a cave’ – showing that by choice he would not mix with human society.
Lennie does not understand the social codes of the world of men, as he shows when he enters Crooks’ room. He is bewildered and afraid of people and gravitates towards animals when he can – collecting the mice and having Slim’s puppy are examples of this.
In a natural setting, and as part of the world of animals, Lennie is accepted. His accidental killings can be attributed to self defence or survival of the fittest. However, in the reality that is the human world- set in the ranch and the bunk house, Lennie is an outcast and a danger.
Lennie reacts poorly to his surroundings for many different reasons. To begin with, Lennie is mentally disabled. They never specify what has happened to him or if he has always been this way, but Lennie has the mental capacity of a small child, maybe around 7 or 8 years old.
Part of the reason Lennie reacts poorly to the surroundings is that George has frightened Lennie into being submissive, or very quiet, about his personality. In this time period, society was not welcoming to people who were handicapped. They were locked away in institutions and typically abused. George threatens Lennie that if he messes up, they will be separated.
Also, Lennie is much like a small child caught around adults who are fighting. He has no clue what is going on around him, but he wants to help or hide. In short, he is being yelled at and harassed and he does not understand motivations or the meanings behind the actions of those around him.
I hope this helps!