When a dog smells your breath it can mean many things but one possibility is that the dog is simply curious about the scent coming from your mouth or your dog is trying to detect any changes in your body chemistry or health. It's also possible that the dog is simply trying to establish a closer bond with you. Dogs use their sense of smell as a way to gather information about the world around them, including the taste of foods and by smelling your breath, they may be trying to learn more about you and your habits.