Why does George lie to the boss about his relationship with Lennie in Of Mice and Men?

lies to the boss because if he realizes that s mental deficiency is a
threat they wont get the job.

George lies and tells the boss
that Lennie is his cousin, and that they left the job in Weed because it was done.  George tells
the boss that Lennie is not smart, but does not tell him hes mentally challenged.

When the men arrive at the ranch, they are already in trouble because they were late. 
The boss is not happy about it.  He asks them questions, but only George answers them.  The boss
begins to get suspicious.

George broke in loudly,
"Oh! I ain't saying he's bright. He ain't. But I say he's a God damn good worker. He can
put up a four hundred pound bale." (ch 2)

The boos
thinks that Gerge might be taking Lennies pay because he is talking for Lennie.  He assumes that
since George seems to be in control, he is taking advantage of Lennie. 

George has to be careful, because if he annoys the boss they might not get the job.  He needs to
walk the fine line of...