What comparisons does Candy make between his own condition and his dog's? How does this relate to the idea of responsibility( mercy killing) ?

Both Candy and his dog are old. One of the themes in this book has to do with the isolation and disposability of people when they are no longer needed. Just as Candy's dog is shot because of his age, Candy feels the same way. He feels old and disposable. Candy laments that he had a responsibility to shoot his own dog instead of letting Carlson shoot it. This event foreshadows the mercy killing of Lennie by George.