Lennie was frustrated when George caught him with the mouse, when Curley was picking on him, and when he accidentally killed his puppy.
Lennie often felt frustrated or insecure because he did not understand what was going on around him. Sometimes he was just lonely, when he wanted something soft to pet. A time that he was both lonely and frustrated was when George caught him with a dead mouse.
George and Lennie were travelling to the ranch, and they stopped by a pond the night before. Their bus driver let them off too early, so they could not get to the ranch in time.
George and Lennie spent a lot of time together, and George took care of Lennie. Lennie was a big man, but he had the mind of a child. First of all, he liked to pet soft things. This is how he ended up with the mouse. He was unhappy when George caught him with it.
"Uh-uh. Jus' a dead mouse, George. I didn't kill it. Honest! I found it. I found it dead."
"Give it here!" said George.
"Aw, leave me have it, George.”
"Give it here!" (Ch. 1)
Of course George did not want Lennie messing with a dead mouse, but Lennie still resented George taking it away from him. He liked to stroke it. He didn’t seem to particularly care that the mouse was dead.
A second example of when Lennie was angry and frustrated is when Curley picked a fight with him. George had warned Lennie not to mess with Curley, because he knew that Curley was threatened by Lennie’s size. However, Curley picked a fight anyway.
Curley decided that Lennie was laughing at him when he caught him smiling. He was really just trying to pick a fight. When Curley attacked, George finally let Lennie fight back.
Suddenly Lennie let go his hold. He crouched cowering against the wall. "You tol' me to, George," he said miserably.
Curley sat down on the floor, looking in wonder at his crushed hand. (Ch. 3)
Lennie was miserable. He did not want to hurt Curley. Once he grabbed his hand, he got scared and could not let go. He crushed Curley, and the reason he hurt him so badly was because he was frightened. Curley decided not to tell anyone what really happened.
The last example of Lennie being lonely and scared, is the puppy incident, which leads to another incident of being lonely and scared, when he kills Curley’s wife. The two incidents are closely related.
Lennie had a puppy. He was stroking it, similarly to the mouse. He accidentally killed it. He was upset about it, and afraid, because he did not want to get in trouble.
"Why do you got to get killed? You ain't so little as mice. I didn't bounce you hard." He bent the pup's head up and looked in its face, and he said to it, "Now maybe George ain't gonna let me tend no rabbits, if he fin's out you got killed." (Ch. 5)
Although Lennie was scared and frustrated about this incident, and his loneliness led to it, because George was outside playing horsehoes, it did not cause him to be more careful and prevent Curley’s wife from getting killed.
Lennie is Lennie. He is basically a child. He does not have any sense of consequence. He acts on impulse. If something it pretty, he strokes it, but he does not know his own strength.
Lennie is protected by George throughout most of his adult life. Although he is sometimes scared, or frustrated, most of the time George can make it better. Unfortunately, after he kills the girl, George can no longer protect him.