Their mutual dependence on one another is what keeps George and Lennie together. Lennie stays with George because he has no other option. His only family, his aunt, has passed away, and he has the mind of a very young child. He would not be able to survive on his own: it is because of George that he is able to find work, and it is George who ultimately cares for him. George stays with Lennie, I believe, out of a sense of duty and an overwhelming loneliness. George promised Lennie’s aunt that he would look after Lennie, and now he has become so used to being with Lennie that he does not know any other way. Lennie, despite the frustration George feels in taking care of him, is George’s only friend.