You know you're reading a bleak book when the most likable (male) character is a dude who abuses his dog.
Salamano is a curmudgeonly old man who lived with his old, disease-infested dog in Meursault's apartment complex. He is the only character in the book who has a complex yet easily understandable and genuine relationship… with his dog. A grumpy, perhaps cantankerous guy, Salamano curses, yanks at, and spits at his dog constantly. In fact, he never shows his true feelings for the dog until it disappears from his life.
Hmm! This odd relationship works in a variety of ways. First, it raises the important notion that man can "get used to anything," a notion later reinforced once Meursault adjusts to prison life. This is a small but important part of the absurdist philosophy. Meursault establishes that to wish for any one life over another is pointless; if any life—even a life with an old dog or one behind bars—is good enough to make a person content, then indeed there is no purpose to striving for anything more.
Secondly, Salamano and his dog remind us of old age and death—that which is waiting for us all at the end of the road (cue the sad trombone). Meursault knows this, but he doesn't accept it until the very end of the novel, when he has his revelation. Check out the descriptions of the dog's scabby coat—it's gruesome for a reason.
Perhaps the most interesting function of this relationship is the fact that Meursault’s big, final revelation—that man and all creatures are made equal by death—is right in front of his face, and indeed ours, from the very beginning of the novel.
Salamano himself says that his dog took the place of his wife, suggesting that animals can operate on an equal level with human beings. Meursault himself notes the old man looks like the dog because they are both old and dying. Salamano and his pet are equal in their mortality—he very revelation we get on the last page.