A dog moaning all the time could mean that they're tired and are moaning because they're tired just like I do myself sometimes.
Sometimes I'm tired and will moan so that could be a reason your dog is moaning all the time or it can also mean that the dog is content and glad to be around you.
Also puppies do moan when they are in close contact to their littermates, their mother, or their humans.
Your dog could also be sick and might have excess fluid accumulating in it's abdomen which makes it groan or moan all the time.
It could be very normal for your dog to be moaning all the time and also your dog might be sick or just is tired.