At first, Crooks is angry that Lennie is trying to invade his personal space, but swiftly it becomes clear to him that Lennie is intellectually impaired, and Crooks allows him in. Talking to Lennie, it is evident that Crooks feels able to tell Lennie anything he wants without fear that Lennie will "blab," and he muses that this is why George must spend time with Lennie. He assesses that it is easy to take advantage of Lennie's simple nature. In testing this, he asks Lennie what he would do if George "took a powder," which would mean he'd go away and not return, leaving Lennie alone.
This upsets Lennie. Crooks suggests that if George were "hurt," they would tie Lennie up "like a dog," but Lennie's instant response is to demand who has hurt George. Crooks sees the danger in this reaction: Lennie is a very big man, and he clearly feels extremely protective of and attached to George. Crooks relents, and he explains to Lennie that George will come back, but that he, Crooks, doesn't have anybody the way Lennie has George, purely because he is black. Crooks is simply using Lennie as a sounding board here, it seems; Lennie is not really interested in what Crooks is saying. But Crooks sees in Lennie an opportunity to talk to somebody, even if that somebody is not really listening.