All quotes used are taken from the Penguin version of "Of Mice and Men" (1978).
The men begin the card game because they are trying to distract themselves while Carlson takes Candy's old dog outside to shoot it: "The way I'd shoot him, he wouldn't feel nothing." (Pg. 45) George does not shuffle the cards: "He rippled the edge of the deck nervously..." (Pg. 49). Candy is laying on his bunk staring at the ceiling. The men know how much that dog meant to Candy, and they are nervous, sad, and feeling sympathetic about Candy losing his companion. They are all waiting in anticipation for the sound of Carlson's shot coming from outside, and they know Candy is too. Everyone is anxious for this difficult scene to be over with.