A Cat is sharing its home with you and whether cats are completely domesticated is questionable. A dog on the other hand is quite domesticated and thinks it is great to live with you, in your home (in particular, inside). Dog's smile with their tails, and occasionally they appear to actually smile, ask any dog owner, he/she thinks theirs does?
The Dog and Cat. When a dog wag's its tail, it's telling all, (the bigger the dog, the more danger to nearby items on the coffee table), that it is happy. A cat's tail has various meanings, as its tail is an extension of its feelings, as well as a warning of the cat’s intentions. Normally the higher the tail, the better the mood. A happy cat will hold its tail straight up, while in fear, it will tuck the tail between the legs, just like a dog does.
A broad swishing indicates annoyance, while twitches are a sign of curiosity and excitement, as when it is ready to pounce on prey. For example, the cat who decides it's had enough petting will signal by swishing the tail in impatience. If you continue, a bat with the paw generally follows. The batting is an act of aggression indicating your cat's agitation. More.....