He barks.  He barks when he's happy, he barks when he's sad, he barks when he's bored and wants my attention, he barks when he's mad.  He barks at people outside, not to scare them; more like, "Hey, get over here and play with me!"  He barks at blowing bags and leaves outside, he barks at imaginary things outside.  He barks at dogs on TV, he barks at noises from the neighbors and at his reflection in the entertainment center.  He barks when he's walking, he barks when he's standing still, he barks when he's hanging out the car window.  He barks.

Case in point: Barking at his reflection in the window.