Now, look- I'll give him the work tickets, but you ain't gonna say a word. You jus' stand there and don't say nothing. If he finds out what a crazy bastard you are, we won't get no job, but if he sees ya work before he hears ya talk, we're set. Ya got that?"  (Chapter 1). 

This quotation shows the reader that Lennie cannot navigate the world without the help of George.  George tells him what to do, and he tells him how to do it.  Without George running interference, Lennie could not even get a job to support himself.  His lack of intelligence would scare away prospective employers, so George has to speak for him in order to help him survive. 

"I forgot, George."

"Yeah, you forgot. You always forget, an' I got to talk you out of it." He sat down heavily on the bunk. "Now he's got his eye on us. Now we got to be careful and not make no slips. You keep your big flapper shut after this." He fell morosely silent (Chapter 2).

In this quotation, George is reacting to Lennie’s inability to remain silent while the boss is talking to them.  Because Lennie forgets things, he is a liability to George, who must make excuses for him and constantly watch out for him.  If Lennie was forced to leave George, he would not survive.  He would not know how to find food or work.  If he found work, he would not be able to maintain the job without George telling him what to do.