I think the difference is that the dog in the glass is outside the boundary of the den—your house—and thus is not recognized as himself. Therefore, barking.

He knows that the glass door leads to the outside and so expects his reflection to be another dog.

I believe that scientific research has found that dogs lack the ability to recognize their own reflection. The question is why your dog doesn’t seem to react to his reflection in a mirror. It is likely some environmental cue that is causing him to be calm with the mirror, but I don’t know what.

Maybe your he is just acclimated more to the reflection in the mirror.

I don’t think he likes going outside. So when he sees himself outside he’s like wuf wuf no I don’t want to be outside – I want to be inside where it’s cozy.

My dog barks at herself in a mirror. We have to keep mirrors out of her sight line

My neighbor swears that her cat can recognize herself in any mirror. My best theory so far is that cats have a degree of depth perception that some dogs don’t have due to the placement of their eyes. My dog, for instance, reacts to his reflection only when he can see through the doorway glass as well as his refection. When it is just his reflection with no depth involved, he doesn’t seem to see himself. I wonder if dogs with front-set eyes will react more like a cat?

It’s not about the eyes, both cats and dogs can see the reflection, but their brains are incapable of recognizing that it is really them.

A reflection in a mirror is recognizable as a dog but a reflection in a window is transparent. The human brain can identify a transparent reflection, probably a dog’s brain can’t