I tend to think that what makes George and Lennie distinct from the others on the ranch is that they are never really isolated. The refrain that both of them recite to one another, like clockwork, is "I got you and you got me." This is something that strikes the other workers on the ranch, the ones who are isolated, as unique. The owner of the ranch thinks that George is taking advantage of Lennie as a reason why they are never isolated from one another. Curley realizes that both men are together and he wants to isolate them when he picks on Lennie and reprimands George for answering. Upon meeting both men in chapter 2, Slim remarks that the relationship both men share is fundamentally different than what is seen around the ranch. Crooks, Candy, and Curley's wife all make mention of how unified both George and Lennie are, almost in stark contrast to the isolation of their own lives.
George and Lennie are not isolated from one another. This is how they endure the difficulties that take a toll on those who are isolated. The idea that George and Lennie have one another as their own form of "fambly" is what demonstrates how they could not be isolated. George has to do what he does to Lennie because they are not isolated. The very idea that Lennie would be tormented by Curley's lynch mob is unacceptable to George precisely because neither of them can be isolated from the other. It is for this reason that Lennie is terrified when he goes back to the brush. His visions of Aunt Clara and the giant rabbit subside when he sees George, demonstrating that he can never be isolated from him and George cannot be isolated from Lennie.