It's because of Candy's dog. I know it's coming and I cannot bear to watch. Candy's dog used to herd sheep but now he's old and he farts and he doesn't see well and he can no longer enjoy his food because his teeth are so bad. But he's all Candy has in the world and he and that dog are inseparable. They love each other. One of the other farm hands convinces Candy that the dog is an eyesore and probably in pain and the only humane thing to do is put it out of its misery. At least then his old, useless life will be painlessly, quickly over. Offstage, he shoots the dog in the back of the head. Candy is filled with remorse because he betrayed his old and true friend and because now he's alone.
Yeah, yeah, I know. I was awake in English class. I know Candy's dog foreshadows the fates of Lennie and George. But I never got as worked up about Lennie as I did Candy's dog.