Here is a classic example of humans misinterpreting animal expressions and body language.

Most if us have seen pictures of captive chimps that have been trained to smile on command. Circus chimps, film chimps and those used in T.V. We tend to accept the big grin as a sign that the animal is feeling secure and is happy enough.
The big toothy grin is, however, how a chimp shows that it is feeling fear - a fear face.
Once trained it does not mean that every time the chimp displays this expression it is fearful, just like a dog can be trained to limp when there is nothing wrong with it. It does show that while the expression is very similar to a human one the meaning behind it is very different indeed.