I'm not sure George exactly "humbles" himself, but he is a quick study of the politics of the ranch and immediately recognizes Slim, "the prince of the ranch," as a person worth cultivating. When Slim questions him about Lenny, George is naturally defensive. It's clear that things are not what they seem at the ranch. Curley's wife -- the "worst piece of jailbait" George has ever seen -- is only the beginning. From the very start, the boss is suspicious of George and Lenny, and George realizes to protect his friend he will have to work hard to stay out of trouble. Even Lenny realizes right away that the ranch "ain't no good place" and wants to leave. So when Slim begins to ask questions about George and Lenny's relationship, George is guarded--when Slim asks about why he travels with such a "cuckoo," George says that he's not so smart himself. But George also senses that Slim might be someone he can talk to:
"If I was bright, if I was even a little bit smart, I'd have my own little place, an' I'd be bringin' in my own crops, 'stead of doin' all the work and not getting what comes up outa the ground." George fell silent. He wanted to talk. Slim neither encouraged nor discouraged him. He just sat back, quiet and receptive.
It is a moment of decision. Should George tell Slim about Lenny? When George tells the story of how Lenny was accused of rape, it's unclear what Slim thinks. He agrees with George that Lenny "ain't mean," but there is also a sense in which George, perhaps foolishly, has spoken a dangerous truth.