The situation with Candy's dog in Chapter Three helps Steinbeck establish two of his important themes in the novella. It is also important because its death foreshadows later events. First, the dog is symbolic of the old and weak who are powerless when faced with a society which views them as useless. Because the dog is old and decrepit, the laborer Carlson contends that it should be put out of its misery. It is no longer useful and its existence is now a burden on the men around it, mainly because it smells bad. Society, in the form of Carlson and Slim, judges the dog and it is ultimately killed by Carlson. Second, the symbol of the dog helps Steinbeck reinforce the idea of loneliness and isolation. The dog is Candy's friend and its loss throws him into a period of depression. He has lost a faithful companion, an animal he has raised and lived with for several years. Only the idea of the dream farm helps Candy overcome the loneliness of losing his dog.
More importantly, the episode with the dog foreshadows the end of the book. When Lennie accidentally kills Curley's wife it is left to George to provide a solution to the problem presented by Lennie's mental disability and continually anti-social behavior. As with the dog, Slim is also part of the judgement against Lennie. He suggests to George that allowing Curley to get to Lennie first or locking him up will not be the right thing to do. In Chapter Three, Candy laments that he should have shot his dog himself instead of allowing a stranger to do what was his responsibility. Like the dog, Lennie has outlived his usefulness. He can no longer continue in society and George ultimately decides that the best thing to do is to kill him rather than allow Curley or the law to do what George sees as his problem. To further emphasize that the death of the dog provides foreshadowing for the death of Lennie, Steinbeck has George use the same gun (Carlson's Luger) to kill Lennie that was used on the dog.