Let Your Dog Sniff and Explore
People have five or six million scent receptors in their noses, and depending on the breed, have up to 300 million. Your do g's brain has a larger area devoted to its sense of smell. Dogs also have a Jacobson's organ, a patch of sensory cells within the main nasal chamber that detects heavy moisture-borne odor particles. The Jacobson's organ helps dogs detect pheromones and other odors. All this means your dog's sense of smell is at least 10,000 times greater than humans.
Dogs experience the world through their noses. Just as we look around to take in the scenery, our dogs smell all their environment has to offer. Dragging your dog away from an exciting scent, or heeling the entire walk, prevents your dog from genuinely taking in everything around him. Not allowing your dog to sniff also diminishes the mental stimulation a walk provides.
Some dogs seem to be ruled by their noses and think of nothing else while on a scent trail. Proper training may help regain their focus when out on a walk. Consider teaching cues like "Watch me" or "Leave It" to take their minds off the smell and put their attention back on you. Reward short bursts of heeling or loose leash walking with frequent sniffing sessions to help foster good walking behavior.
(Information from AKC)