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In John Steinbeck's book, Of Mice and Men, Lennie isn't lonely, but is actually more afraid of becoming lonely if George leaves him.
Lennie is tall and powerful, but he has the mind of a child. He travels with George who tells people they are related. George had promised to care for Lennie when his last living relative (Aunt Clara) passed away. George gets impatient with Lennie, but still very much cares for him, suffering the inconvenience and even fear that comes from dealing with a man who is mentally challenged and very strong.
Lennie does not know his own strength, and acts inappropriately sometimes—with the innocence of a child. This often causes problems and the two find themselves on the run. (At their last job, he wanted to touch a girl's soft dress; she became frightened, and eventually they were run out of town.) They are forced to travel as many people did at that time, due to lack of work during the Great Depression.
Lennie has George and believes that each one of them helps the other by providing the other with companionship and someone to talk to. (Though George comforts Lennie with this idea, for George it is not true.) However, Lennie, does have someone who takes care of him, who he can talk to. Although Lennie does not fit in with others, he has George and tries very hard not to make him angry.
Lennie's problem is not loneliness; rather he wants to be able to touch soft things. At the start of the story, Lennie carries a dead mouse in his pocket and George has to take it away from him.
George took the mouse and threw it across the pool to the other side, among the brush. 'What you want of a dead mouse, anyways?'
'I could pet it with my thumb while we walked along,' said Lennie.
But Lennie is determined to have the mouse. George sends Lennie to get wood, but while he is away, George hears Lennie splashing through the water, knowing that the man is searching for the dead mouse.
'...I want that mouse.'
Lennie...reached into his pocket...'I don't know why I can't keep it. It ain't nobody's mouse. I didn't steal it. I found it lying right beside the road...I wasn't doin' nothing bad with it, George. I was jus' strokin' it.'
George explains to a tearful Lennie that he isn't trying to be mean. Lennie has crushed it with petting it, and it isn't "fresh" anymore. He promises that the next time he finds a mouse, George will let him keep it for a time. This incident foreshadows a very serious event at the end of the story as Lennie wants to play with a puppy.
In the second to the last section, once again Lennie has tried to play with an animal: this time it is a puppy. Whereas the mouse he found earlier was dead, this time the animal is alive; but Lennie is too rough and kills the pup. He is saddened, and afraid that now George won't let him tend the rabbits when they get their house.
Curley's wife comes in and starts to speak to Lennie. He tries not to talk with her, having been warned by George, but she is insistent. When he explains his love of soft things, she puts his hand on her hair. When he won't let go, she becomes terrified and screams. To keep her quiet, fearful that George will get mad that they spoke, he covers her mouth, but in the struggle he breaks her neck.
Lennie's problem is not one of loneliness, but more a fear of losing George and then being lonely.