Maybe you want them to do it, and maybe you haven’t given it much thought, but there your dog is, rolled over on his back, tongue lolling out of his mouth, happy as can be. Why does he do it? Is it for belly rubs, because you taught him to do that or is it something else entirely? There are actually a number of reasons it could be, actually.
The first reason is rather obvious; because you trained them to do it. Dogs are intelligent and usually take well to training, even if the task isn’t entirely natural to them. Training a dog to do something such as rolling over is pretty simple. Dogs are creatures that are eager to please, and they know that doing tricks for you makes you happy. Whether it is because they want to treat reward for it or they want your attention, dogs roll over for you.
However, while this may be true, there is also the fact that you cannot train a dog to do something that does not have roots in their natural behavior. While teaching a dog to, for example, open a door may seem a bit far fetched with that idea, it stems from natural behavior: tugging. Dogs rolling over is also a natural behavior, though the reasoning may be slightly different for wild dogs than they are for domesticated dogs.
For dogs in the wild, such as wolves, rolling over is a sign of submission or forfeit. Often times during a fight, whether it is a play fight or a real one, dogs will roll over as a sign of submission. If it is a play fight, they are doing it for fun, acknowledging to the other dog that their game is just that, and also acknowledging that the other dog is the alpha of the pack.
When they roll over in the middle of a real fight, it is a sign that they forfeit and are accepting the other dog as the winner. Doing this exposes their belly and gives their life into the “hands” of the winning dog, so to speak.
So training a dog to roll on its back stems from its acknowledgment that you are the alpha of the pack. When they are rolling over for you, they are showing their respect for you and telling you that they are submissive to you. When a dog shows that they are submissive to you, especially if it is not a fearful dog, they are showing that they trust you.
Even when the dog rolls over in the hopes of a belly rub, they are also showing that they trust you. In the wild, exposing their underside is a very dangerous action. They are trusting you not to attack them, and they are wanting the contact with you.
Dogs enjoy physical contact with one another, and with their owners as well. When they roll over for a belly rub they are not only showing their submission and trust, but they are saying that they want affection. Especially if you have taught them that this is a desired behavior and they will get rewarded with a belly rub and perhaps a treat.
Not all dogs will readily roll over, however. Some do not want to learn the trick or simply cannot learn the trick. Whether it is because of a trust issue the dog has (they don’t want to expose their belly to you, a family member, or another pet), or because it is too uncomfortable for them (in which case it may be a good idea for a vet to check them out as this could be a medical issue), or something else entirely, not all dogs can be trained to roll over and not all dogs will roll over for no reason.
If you are concerned it may be a medical issue that causes your dog not to roll over, check with a vet to make sure. It could be as simple as the dog has a little too much fat to make it comfortable for them. If you are concerned that it may be a trust issue, watch your dog for other behavioral indicators and see a dog behaviorist. Otherwise, there is no need to worry. And if you dog is rolling over on its back every chance it gets, make sure to reward it with a good belly rub.