George has been angry at Lennie ever since that incident in the town of Weed. Lennie molested a girl on the main street in broad daylight, and the two men had to run for their lives. George is only expressing his anger now. This happens all the time between people. It often happens between married couples, for instance. One or the other explodes and vents a whole lot of issues and grievances in one harangue. George probably hasn't told Lennie off before because he knows Lennie is mentally handicapped and isn't responsible for his behavior. Furthermore, George doesn't think Lennie could understand him anyway. George took on a heavy burden when he promised Aunt Clara he would look after Lennie. For how long? He could be tied to this feeble-minded giant for thirty or forty years. George is like a volcano that will erupt periodically. The anger is always there because the burden and the resulting resentment and frustration are always there. Lennie is more of a liability than an asset. George is a "little guy" and Lennie may afford him some protection, not unlike a big watchdog, in the harsh world where men travel on freight trains, sleep in the open or in hobo jungles, and have to compete for low-level jobs as unskilled laborers. But Lennie is always getting them both fired, if not getting them into more serious trouble. Lennie's guilt rubs off on George, because has assumed responsibility for Lennie and knows Lennie is capable of causing trouble wherever he takes him. George's eruption of anger in the opening scene foreshadows what is bound to happen sooner or later. George wants to be free of his burden, whether he is conscious of the fact or not. When he finally kills Lennie in the last chapter, he has mixed feelings. Naturally he feels guilt, sorrow, and pity; but he also feels relieved. People keep changing. Friendships don't last forever.
The first scene of Of Mice and Men serves as the exposition to John Steinbeck's novella of the social plight of the disenfranchised men of the Great Depression. Into a quiet clearing outside Soledad, two solitary men enter, the larger one following the other man in the shuffling, lumbering gait of a bear. After they set up camp, George Milton, the smaller man, cooks three cans of beans on the fire made from wood gathered by Lennie.
"There's enough beans for four men," George said.
Lennie watched him from over the fire. He said patiently, "I like 'em with ketchup."
Angrily, George informs Lennie that there is no ketchup, complaining that whatever Lennie wants, they do not have. He then launches into a tirade about how much trouble Lennie is and how he could get along so much better without him.
"I got you! You can't keep a job and you lose ever' job I get. Jus' keep me shovin' all over the country all the time. An that ain't the worst. You get in trouble. You do bad things and I got to get you out.....You keep me in hot water all the time."
This angry complaint foreshadows what occurs later in the narrative as Lennie does, indeed, get them into "hot water" with Curley when he crushes the shorter man's hand and on the ranch when he inadvertently breaks the neck of Curley's wife, who has flirted with him, asking him to touch her silken hair as she taunts him in the barn when all the other men are gone to town.