Wanting a walk.
Last year, my honors candidate had a small cat she and her girlfriend had rescued, and it was living in her room. When I'd ask her how her cat was, she'd say, "She's a little mouthy"--and then show me her various scratches and welts.
This dog can get a little mouthy when he's ready for his walk, but fortunately he confines his mouthiness to barking and snapping at me. In fact, now, sometimes when we're about to go for a walk, he'll just step into his harness. But a good part of the time, he still looks something like this.
Or this (exactly seven seconds earlier than today's first picture):