That's true in many cases but not always.  (on the baring teeth)  When I come home, Emma is all butt tail wag and smiles.  She even responds when I say, "smile".   My mom had a dog that smiled too.  

Dogs know when they're in trouble just as a toddler would.  They don't like that feeling any more than a human does.  My Emma is the smartest damned dog I have ever seen. When they were both in their dog run they were getting out somehow.  To see where they were getting out, my husband put Emma in, walked away and called her.  She wouldn't move.  So he put Molly in.  Molly doesn't have Emma's intelligence at all so when he called her, she showed him exactly where they were getting out.  Emma knew better.  She's even plotted her escape from the property, digging closer and closer to the fence as the battery in her electronic fence collar went low over several days.  If that dog had opposable thumbs...     :)

...... Kendra