There could be several reasons why a dog might be mouthy with their owner. One common reason is that they have not been taught proper bite inhibition from a young age. If a puppy didn't have sufficient opportunity to socialize and learn appropriate play behavior with littermates, they may not have developed bite inhibition skills. Additionally, some dogs may resort to mouthing as a way to get attention or engage in play with their owner if they haven't been provided enough mental or physical stimulation throughout the day. It's important to establish consistent boundaries and redirect them towards appropriate behaviors.