Far as I can tell, it just hurts their delicate sensibilities, lol.
I always brush about the same time every night. She's finally gotten to the point where...all on her own...she slooowly crawls up on my lap, ears down, with that "please don't beat me tooo hard" expression. Then she just sits and sorrowfully waits for the brush. They don't come much sweeter than my little Zo.
I'm just brushing the outsides of her teeth...I read somewhere that that was enough. Does anyone brush the insides, too?