Why could Lennie not understand George leaving him for good?

Lennie and George have a very strange relationship. George constantly tells Lennie how worthless he is, and how easy it would be for George to just leave him. But George never left Lennie, and they stick together like glue. One night, when George is away and Lennie is staying behind on the ranch, he goes into Crooks room. Crooks and Lennie start to talk when Crooks says, "'S'pose George don't come back no more. S'pose he took a powder and just ain't coming back'" (71). Lennie simply just cannot comprehend that happening. I believe because he has stayed with George so long, he has become a huge part of life. To Lennie, George leaving him is like humanity going extinct the following day. Lennie has always stayed by Georges side, under his protection. Despite Crooks trying to see what Lennie would do without George, Lennie continues say things like, "'George wouldn't do nothing like that'" (71). Lennie simply cannot understand George leaving him.