Lennie is dependent on George because he is mentally challenged.
Lennie is a big, strong man, but he is not mentally strong. He relies on the smaller, quicker George. Georgie looks after him and takes care of him. If it weren’t for George, Lennie would be locked up or dead. He does not know not to drink scummy water, and he gets in trouble for attacking a woman because he wants to pet her dress since it’s soft.
When George introduces himself to the ranch boss, he explains why he looks after Lennie.
“… I knowed his Aunt Clara. She took him when he was a baby and raised him up. When his Aunt Clara died, Lennie just come along with me out workin'. Got kinda used to each other after a little while." (Ch. 3)
George explains that Lennie is not crazy, but he is “dumb as hell.” He has the mind of a child. He goes after he wants, knows nothing of consequences, and always has to be told what to do. He respects and loves George. George guides him and looks out for him, and keeps him in a job. If he wasn’t going around with George, he could never be employed. No employer would take the risk. George vouches for him and takes the risk so the employer does not have to. When something bad happens, they leave. This is what happened in Weed, when Lennie tried to touch the lady’s dress. They left.
In the end, even George can’t protect Lennie. He can’t be with him all of the time. Curly’s wife is so lonely that she goes up to Lennie, and Lennie strokes her hair. He accidentally kills her. Lennie did not mean it, but George realizes that the only way to really protect Lennie is to kill him.
"Yeah," said George. "I'll come. But listen, Curley. The poor bastard's nuts. Don't shoot 'im. He di'n't know what he was doin'." (Ch. 5)
George knows that if the men caught him, they would do far worse. He also knows that if the law caught up to him, Lennie would not understand what was happening to him. He tries to protect Lennie in the only way he can.
Lennie is dependent on George due to his mental state of being very child-like. He copies George numerous times throughout the play, such as the manner in which George is speaking, sitting, standing, etc. He looks up to George and sees George as not just a friend, but a protector, and someone he can emulate. Moreover, Lennie is protective of George as well (see the scene with Crooks in his room, where Crooks surmises that George is dead and not coming back). Lennie, raised by his Aunt Clara until her death, is unable to fend for himself, as George suggests when Lennie says he will live in a cave by himself. Therefore, he needs someone like George to ensure he is safe and does not do any "bad things" unintentionally (such as the incident in Weed with the girl in the red dress). Lennie does not know his own strength and is capable of doing much harm if left unchecked and unsupervised - as seen with the puppy and Curley's wife. Without George, Lennie would probably have ended up in an insane asylum or jail and eventually died. Lennie needs George in order to survive in the harsh reality that is America during the Dust Bowl. He would never be able to obtain a job or find a living situation without George to speak for him and keep him out of trouble. George is Lennie's only option of survival in life.
Lennie is dependent on George because George is the only person who enables him to survive without being institutionalized. George gets Lennie jobs. The jobs provide Lennie with food to eat and a bunk to sleep in. Furthermore, the jobs pay some cash wages. Steinbeck does not show how Lennie spends his money, but he does have money to spend on necessities like shoes and on little luxuries like candy and ice cream. Without George, Lennie would not be able to get a job. He would immediately become destitute and most likely would be committed to some kind of mental asylum. The main such institution in California in those days was in Napa. Mentally retarded men were not cruelly treated there. A man like Lennie who could do hard outdoor work would be assigned to work at nearby farms and vineyards. Napa is in the heart of the California wine country, and there is always a need for labor. That is, if Lennie would conform to the rules and not try to run away. If he did run away, he would probably end up back in Napa and kept locked up. But Lennie might be satisfied with institutional life which provided a bed, clothing, hot meals, and some companionship with others like himself.
Lennie depends on George because he really does not know how to comprehend things fully. Lennie has the mindset of an eight year old child which makes him vulnerable to his surrounding's so much and this is why George takes care of him like a little child. Also there is the fact that Lennie and George grew up together so you can say George has been the authority figure for Lennie, especially when Lennie's aunt Clara died.
Lennie is so dependent on George because he is not mentally stable and independent. This means he needs support from his family who is George. For George this is a painful and harsh experience as he struggles to control and maintain his stable behaviour and not get him into trouble. Therefore Lennie is quite troublesome for George.