There are two reasons - 1) Lennie is a responsibility to George, and George takes that seriously. 2) Lennie is a companion to George.
George made a promise to Lennie's dying aunt to take care of him. George is an honest man and a loyal one. He will not break his promise. We know this in the first chapter when he explains how hard it is for him having to care for Lennie:
I could go get a job an’ work, an’ no trouble. No mess at all, and when the end of the month come I could take my fifty bucks and go into town and get whatever I want . . . An’ whatta I got,” George went on furiously. “I got you!”
And yet, despite his anger, he won't give Lennie up.
The second reason is the companionship. A major theme in this novel is that of loneliness and alienation. The life of these migrant workers during the Depression was a sad one. This is demonstrated through the "handicaps" of the characters - Lennie's intellect, Candy's hand, Crook's skin color, etc.. However, Lennie and George have a friendship these other characters don't. This is why Candy wants to join with them in the plan to buy a little farm. He wants to have a group, too.
"Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place....With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us."
These two men are united in their struggle to escape poverty, loneliness and hopelessness. Although Lennie is mentally handicapped, he is a special friend to George, more like a brother.
Lennie needs help because although he is a full grown adult, he has the mind of a child. In addition to his child-like innocence, Lennie possess a powerful strength that constantly gets him into trouble.
George's life has more meaning because he acts as Lennie's caretaker, having promised his Aunt that he would look after him. They talk and share a sweet, never-going-to-happen, vision of being their own bosses someday on a farm that they would own together.
"George and Lennie dream of owning a farm, but by the end of the novel the dream has failed. Their plan is doomed because human fellowship cannot survive in their world and also because their image of the farm is overly idealized."
Lennie is a challenged friend of George's. It would seem that George would be more important to Lennie but this can be seen two ways. First, of course George provides Lennie with the opportunities to live a full life. But, how is Lennie important to George?
These men spend their days traveling from ranch to ranch searching for work. This is the only way they can escape poverty. Interestingly, John Steinbeck worked as a farmhand in his early days to help make ends meet. It is not surprising that this story is so realistic!
Lennie is George's only "family". George has real meaning in his life because he has to keep Lennie safe. Lennie is the soul of George's dreams. Never once does George think of getting a house on his own. He always considers the American Dream and how he and Lennie will someday save enough money to buy their own place so they can escape the hard life. Lennie is George's reason to keep living.
It is ironic that George has to make an almost impossible decision at the end of the book. Ultimately, he makes a decision to protect Lennie until the end even when in doing so, he destroys his own future.
lennie is mentally challenge, so george and lennie have known each other for a long time, and george tends to help lennie with this. also lennie and george want to buy a house for them two and so they need each other to buy the house. plus lennie is big and strong and george is not so george needs a guy like lennie