Why did George order Lennie to leave Curley's wife alone?

There are two reasons why George ordered Lennie to stay away from Curley's wife. 

First, Lennie does not know how to interact with people.  Therefore, people can misunderstand him, which could cause trouble.  This is especially important, because Curley is the boss's son.  We should also note that George and Lennie had to flee in the past, because of a misunderstanding between Lennie and another woman.  In connection with this, Lennie does not know his...

There are two reasons why George ordered Lennie to stay away from Curley's wife. 

First, Lennie does not know how to interact with people.  Therefore, people can misunderstand him, which could cause trouble.  This is especially important, because Curley is the boss's son.  We should also note that George and Lennie had to flee in the past, because of a misunderstanding between Lennie and another woman.  In connection with this, Lennie does not know his own strength.  So, an accident can happen. 

Second, George does not trust Curley's wife.  For example, the men were saying that she had the eye.  Here is the dialogue:

“Yeah? Married two weeks and got the eye? Maybe that’s why Curley’s pants is full of ants." “I seen her give Slim the eye. Slim’s a jerkline skinner. Hell of a nice fella. Slim don’t need to wear no high-heeled boots on a grain team. I seen her give Slim the eye. Curley never seen it. An’ I seen her give Carlson the eye.”

George does not want any trouble.  Hence, it is better if Lennie stays away.  He was right.