There are a few examples in which Lennie feels frustration, anger, relief, loneliness, fear, and humiliation. His emotions are interesting because they contrast with emotions and reactions usually portrayed by adult men. Because Lennie has a less able mental capacity, his emotions can be different than "normal" reactions.
Frustration -- Lennie feels frustration in chapter one when George will not allow him to keep the dead mouse in his pocket to pet. Despite the fact that Lennie insists he didn't kill it, George forces him to get rid of the dead animal, and Lennie becomes frustrated--though he does eventually follow directions:
"What you got in your hand—hidin’ it?”
“I ain’t got nothin’, George. Honest.”
“Come on, give it here.”
Lennie held his closed hand away from George’s direction. “It’s on’y a mouse, George.”
“A mouse? A live mouse?”
“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!” Lennie’s closed hand slowly obeyed.
Anger -- One example in which Lennie feels anger is in his scene with Crooks. Crooks, trying to get Lennie to understand his loneliness, asks Lennie to consider what would happen if George never returned from his night out on the town with the other guys. Lennie is unable to think hypothetically and gets angry, thinking the Crooks has or will harm George:
Lennie stood over him. “What you supposin’ for? Ain’t nobody goin’ to suppose no hurt to George.”
Crooks removed his glasses and wiped his eyes with his fingers. “Jus’ set down,” he said. “George ain’t hurt.”
Lennie growled back to his seat on the nail keg. “Ain’t nobody goin’ to talk no hurt to George,” he grumbled.
Relief -- Lennie feels relief at the end of the novel when he realizes that George isn't angry with him and will still allow him to tend the rabbits on their fictional ranch. This is an important emotion because it is George's final gift to Lennie. Instead of Lennie being frightened by Curly, George protects him by keeping him calm and killing Lennie swiftly when he is filled with peace and relief.
Loneliness -- A time when Lennie feels lonely is after he has killed Curly's wife and he's waiting for George to find him at the place they decided to meet. The reader can also assume that Lennie is lonely when he seeks out Crooks in chapter four when George is out with the other men in the group.
Fear -- One example in which Lennie feels fear is when Curly's wife begins to scream while Lennie is touching her hair. Lennie doesn't understand why she is screaming and how he can make it stop or fix the situation, so he becomes very fearful of reprisal from George and inadvertently snaps her neck:
“Look out, now, you’ll muss it.” And then she cried angrily, “You stop it now, you’ll mess it all up.” She jerked her head sideways, and Lennie’s fingers closed on her hair and hung on. “Let go,” she cried. “You let go!”
Lennie was in a panic. His face was contorted. She screamed then, and Lennie’s other hand closed over her mouth and nose. “Please don’t,” he begged. “Oh! Please don’t do that. George’ll be mad.”
She struggled violently under his hands. Her feet battered on the hay and she writhed to be free; and from under Lennie’s hand came a muffled screaming. Lennie began to cry with fright. “Oh! Please don’t do none of that,” he begged. “George gonna say I done a bad thing. He ain’t gonna let me tend no rabbits.”
Humiliation -- One example where Lennie feels humiliated or embarrassed is in chapter one. George has drilled him many times regarding where they are headed, but Lennie forgets after only a few minutes. Lennie is embarrassed because he knows he should remember, but he can't help forgetting:
"You remember where we’re goin’ now?”
Lennie looked startled and then in embarrassment hid his face against his knees. “I forgot again.”