When dogs wag their tails, it’s an energy indicator. They're excited by something. That may mean they are happy or feeling affectionate. It may also mean they're getting ready to eat, or they're looking forward to a walk. We humans tend to interpret all that energy as happy, and it often is.
But studies have shown that dogs also wag when they're not happy. A low timid wag might mean it's frightened. A wag that's cocked to the left might indicate it's being cautious, and a human may want to think twice before reaching out to pet it.
The same wagging patterns are not seen in cats. A cat's tail suggests something is going on, and usually the something isn’t good.
For example, when a cat’s goal is thwarted, as when it's blocked from reaching a bird outside a closed window, the cat will switch and flick its tail from side to side like a whip in frustration. It looks agitated, because it is agitated.
For the most part cats’ tails are saying something along the lines of agitation, frustration, mild annoyance, thwarted opportunity, and so on. The only satisfied wag you might see in a cat is a gentle side-to-side sway, not really a wag the way dogs wag.
