What gives candy the confidence to argue with curleys wife?

Candy's dream of a future gives him a courage he didn't previously have. Never before had he looked ahead and felt secure about what the future had to offer. Prior to his interactions with George and Lennie about the farm he had no future..... if he'd lost his job he wouldn't have had anywhere to go, or anything to go to. Thus, Curley's wife's words aren't threatening to him, and the security he feels about the men buying the farm and working together allows him to speak up for himself.