Fido just can't help himself. Each time you scratch that sweet spot on his belly, his hind leg starts kicking like crazy--but why?

Ultimately, it's all about self-preservation.

"Dogs kick when we scratch their belly because it's an involuntary reflex," Dr. Marc Bekoff, a canine expert based in Boulder, Colorado, and author of the book "Why Dogs Hump and Bees Get Depressed," told The Huffington Post in an email.

And since a scratching or tickling sensation could actually be a dangerous parasite or irritant, such as a flea, the reflexive kicking is a sort of built-in mechanism intended to flick the offender away, according to a recent article in Popular Science.

So those kicks are akin to certain reflexes of your own.

“Let’s say you touch a hot stove, and before your brain recognizes it’s painful, the spinal cord recognizes the pain, and you involuntarily jerk your hand back,” Dr. Lore Haug, a veterinarian and animal behavior expert for Texas Veterinary Behavior Services, told Popular Science. “If you had to wait until your conscious brain recognized something was in danger, your delay in reaction time could cause an injury or even death in some cases.”

In fact, veterinarians may use a dog's scratch reflex to diagnose neurological problems -- a bit like how your doctor might use a rubber hammer to test your knee-jerk reflex.

Now you know.