Why does George stay with Lennie in the novel Of Mice and Men?

George stays with Lennie, because they are friends.  This might sound like a simplistic answer, but it is actually profound in the world of Of Mice and Men. In this world, there are very few friendships.  Alienation reigns supreme. All the men on the ranch are alone.  Even the men who have been on the ranch for a long time are alone. Candy is alone; he only has his dog. Crooks is completely alone...

George stays with Lennie, because they are friends.  This might sound like a simplistic answer, but it is actually profound in the world of Of Mice and Men. In this world, there are very few friendships.  Alienation reigns supreme. All the men on the ranch are alone.  Even the men who have been on the ranch for a long time are alone. Candy is alone; he only has his dog. Crooks is completely alone as a black man.  In fact, he has had no visitors from the other men.  Lennie was one of the first people to visit him.  Curley and his wife have a horrible relationship, even though they have been married a few weeks. 

Within this context, George and Lennie has something special and amazing - companionship. 

Here is conversation between Slim and George that makes this point:

Slim looked through George and beyond him. “Ain’t many guys travel around together,” he mused. “I don’t know why. Maybe ever’body in the whole damn world is scared of each other."

“It’s a lot nicer to go around with a guy you know,” said George.

In conclusion, one of the main themes in the book is the friendship that Lennie and George share together.