Dogs laying on their back?

It's natural when a dog feels safe and comfortable in their environment. The belly area is a vulnerable area to expose for a lot of animals. All the dogs in my life have exposed their bellies many times inside the home. Either when sleeping or when wanting a belly rub. It can also be a submissive gesture, depending on the situation.

Young puppies do it, so you know it is natural. Not all breeds are comfortable doing it as they become grown up but many dogs will lay on their backs off and on throughout their lives.

Think of it this way: Maybe you almost always sleep on your back but don't you sometimes like to rest on your side or even your stomach? It's just another way dogs can lay down that feels good to those that do it.

Dogs laying on their back?

We don't lay on our backs as a defensive or submissive posture so not from us. It's a way (for canines) to expose their unprotected under bellies and show respect for elders/ dominates.

It's natural. Some breeds are more prone to doing it, and dogs that do it are comfortable in their environment (exposing their bellies).

its a submission to its leader of the pack,

when dogs fight to a standstill the weakest will lay on its back as a signal of submission

The bare skin on their tummy absorbs sunshine and manufactures vitamin D..those that didn't do it are now extinct..

It is a submissive posture. A dog that is happy to show you its belly regards you as a trustworthy pack leader.