George is constantly explaining himself and his relationship with Lennie. He feel that people are owed an explanation as to why he travels with this man who "ain't too smart". He tells Slim how he came to travel and look out for Lennie. He talked about the promise he made to the old lady to look out for him. He also says that he feels he could get a lot more done and probably have a better life if he could travel without Lennie, but he keeps him around out of a feeling of obligation and perhaps guilt at the way he has treated Lennie in the past. George can get easily frustrated with him because Lennie has made it difficult for either one of them to hold a job in one place for too long. George's frustration can easily turn to guilt because Lennie doesn't know any better and therefore shouldn't be abuse, physically, emotionally, or otherwise.