why does lennie offer to run off and live in the hills ?

In Chapter one of Of Mice and Men George and Lennie are making camp. Lennie wants some ketchup with his beans but they have none. George gets frustrated about Lennie wanting what they don't have and launches into his familiar speech,

"If I was alone, I could live so easy.

I could get a job and work and no trouble.

And when the end of the month come, I could take my bucks,

I could go into town, I could get whatever I want.

I could stay in a cathouse all night....."

Lennie threatens to run off into the hills, "George, if you don't want me, I go off in them hills and get a cave. And I wouldn't get no mice stole off me either."

This routine seems like it has been done before. Both men are operating at their respective levels of understanding. Although an argument, there are undertones that both are never serious. It is a predictable form of venting that each does and then the relationship is back to normal.