George has been angry with Lennie ever since they had the trouble in Weed. This is the first occasion, however, in which the author has an opportunity to present a lot of exposition in the form of an angry diatribe. It should be kept in mind that Steinbeck intended to adapt his story into a stage play almost immediately. The book and the play came out the same year. Steinbeck intentionally used dialogue for exposition in the book because that made it easy to convert the book into a play. In a play there is no such think as exposition. The audience has to learn everything from what the characters say to each other. The audience even has to learn who these characters and what their relationships are from what they say. The exception is in a play like Our Town in which there is a narrator explaining everything to the audience.
Steinbeck chose to have George become angry at the campsite in the first chapter because the anger makes the scene more dramatic. But mainly it can be seen that George is explaining all about their relationship and about what happened with the girl in Weed. It is purely exposition, not real anger. George gets angry because Steinbeck makes him get angry. Steinbeck makes him get angry because he wants his character to present a lot of essential information but at the same time to make it feel dramatic. There is no real conflict between George and Lennie. Lennie offers to go off and live by himself, but George tells him to stay. When the blow-up is over, their relationship is exactly the same as it was before.
What happened in the little town of Weed is of great significance. Lennie is starting to take an interest in girls. George doesn't realize that Lennie is dangerous until he sees the body of Curley's wife in the barn. It should be noted that George doesn't actually see what made the girl in Weed start screaming. He was some distance away. He has only Lennie's account of the incident--and Lennie is always lying to George, as he lies about the mouse in his pocket. It is also noteworthy that George is not present when Lennie ends up killing Curley's wife. George only sees the dead body and assumes that Lennie was trying to rape the girl and accidentally killed her.
Unlike George, who is consciously aware of his environment, Lennie is unaware of his surroundings and his behavior in general. Upon entering the clearing, Lennie almost walks over George. After entering the clearing, Lennie and George come to a pool and Lennie plops himself down and begins to drink too fast. George scolds him because he doesn't want Lennie to get sick. George scolds Lennie quite a bit in this novel. At times, it is out of exasperation. But most of the time, George is just trying to get through to Lennie to make sure he doesn't hurt himself or someone else. George is frustrated with Lennie because they had to leave their last job prematurely. George is also frustrated because he has to continue to repeat their plans for the future:
"That ranch we're goin' to is right down there about a quarter mile. We're gonna go in an' see the boss. Now, look- I'll give him the work tickets, but you ain't gonna say a word. You jus' stand there and don't say nothing. If he finds out what a crazy bastard you are, we won't get no job, but if he sees ya work before he hears ya talk, we're set. Ya got that?"
Since Lennie forgets, George feels compelled to continue telling him not to talk and to avoid any chance of an awkward social situation.