People who hike with dogs get very defensive when you tell them why it's a bad idea. For everyone's reference, here's what a National Park website says about it.
Why Are Dogs Prohibited on National Park Trails?
Large national parks that have extensive backcountry areas as a rule do not allow dogs on trails. These include parks such as Yellowstone, Yosemite, Grand Canyon, Glacier, Rocky Mountains, and several others. Great Smoky Mountains National Park has prohibited dogs in the backcountry since the park was first established in the 1930s. The park prohibits dogs on hiking trails for several reasons:
• Dogs can carry disease into the park's wildlife populations.
• Dogs can chase and threaten wildlife, scaring birds and other animals away from nesting, feeding, and resting sites. The scent left behind by a dog can signal the presence of a predator, disrupting or altering the behavior of park wildlife. Small animals may hide in their burrow the entire day after smelling a dog and may not venture out to feed.
• Dogs bark and disturb the quiet of the wilderness. Unfamiliar sights, sounds, and smells can disturb even the calmest, friendliest, and best-trained dog, causing them to behave unpredictably or bark excessively.
• Pets may become prey for larger predators such as coyotes and bears. In addition, if your dog disturbs and enrages a bear, it may lead the angry bear directly to you. Dogs can also encounter insects that bite and transmit disease and plants that are poisonous or full of painful thorns and burrs.
• Many people, especially children, are frightened by dogs, even small ones. Uncontrolled dogs can present a danger to other visitors.
