The first possible reason, and the seemingly most obvious, is that your dog has an itch to scratch. After all, humans do it when the itch is unreachable — we look for a sharp wall end or door frame to wiggle on.
Most people know that dogs will pant or expose their tongues to cool off when they are hot. The belly, which usually has lighter fur, can also be exposed to offer additional relief.
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Some dogs love their humans. When they see you, they incline to show you that they welcome your presence and are no threat to you. So they roll over onto their backs, wag their tails, and squirm and wiggle.
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When a dog hunts in the wild, it sometimes needs to mask its scent in order to stalk prey. It would therefore roll around in anything that had a more potent smell.
They have learned that lying on their backs can effectively lure innocent “prey” to the “kill.” This type of behavior is usually not desired and displayed by typically anxious dogs.
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