Quote 1
The old man [Candy] squirmed uncomfortably. "Well-hell! I had him so long. Had him since he was a pup. I herded sheep with him." He said proudly, "You wouldn't think it to look at him now, but he was the best damn sheep dog I ever seen." (3.56)
This is almost the exact same thing that George says about Lennie: he's "had him so long." But can you really be friends with a dog? Or someone who's way mentally inferior to you?
Quote 2
"A guy on a ranch don't never listen nor he don't ast no questions." (2.67)
Candy gives us a pretty good definition of an isolated person: someone who doesn't ask questions and someone who doesn't listen—in other words, not much of a conversationalist. We're getting the feeling that, for Steinbeck, isolation is mostly about silences… which makes friendship mostly about conversation.
Quote 3
George half-closed his eyes. "I gotta think about that. We was always gonna do it by ourselves." Candy interrupted him, "I'd make a will an' leave my share to you guys in case I kick off, 'cause I ain't got no relatives or nothing…" (3.218-219)
Candy is so isolated that he doesn't even have relatives to leave his money to.