His master's horrible voice.

The title of the YouTube video is "German Shepherd Dog sings with his owner," but I'm not convinced that's what's going on here. Oh, he's chiming in. It just doesn't sound a lot like like singing. And that goes for both of them. There's a good chance that what the dog is doing isn't a crude attempt at harmony, but rather, it's a desperate cry for help in the belief his owner's wailing could be the result of a mortal wound. Most likely, he just wants the guy to stop.

Sources: Bro Bible