My reasons are as follows (though I may start out on a track that you just do not see a connection for, I swear it is relevant).
I have raised cats for many years. I started out when I was somewhere around 7 or 8 I'd venture to guess. I will nurse sick cats, I have bottle fed kittens from only 4 days old, all the way to adulthood when their mother abandoned them.
I say that cats smile. When I say this, I get people look at me as if I were insane. "Cats don't smile" they'd tell me. Well, I say "why not?"
If I were to say that my dog gets excited when I come home, I don't get too many weird looks. This is because most people have seen dogs when they are excited. They run around the room, they wag their tail, they bounce all over the place. So to say that a dog is excited, is not weird. So how would you classify this communication from the dog to you. In other words: how do you know the dog is excited? Does the dog come up to you and say "I'm excited"? No, of course not, dogs don't speak, at least not in a human language. So I would personally have to classify that the dog told me that he was excited through body language. He's running around all hyper and all, so he is communicating through his body language.
Ok, so then what is the difference between the body language of a dog running around and acting excited, and my cat smiling? Wouldn't they both be clasified as body language? A smile is much more subtle than bouncing all over the place. So would it be fair to say that maybe it is possible that my cat can smile, but that it is too subtle for some to understand? I have been dealing with cats for many years, maybe this will make me more qualified to say that cats can smile than someone who once owned a cat. When they were a kid. Or someone that doesn't really deal with a cat too much. Maybe I could be better at the judgement?
So it is possible that there can be forms of emotions and expressions performed by other species that may not be identifiable by humans (or at least not all humans). In other words, that we as a race don't know everything.
Well, how about plants? How would we be able to identify if a plant has emotions? How would we be able to identify between it's moods? Well, we wouldn't now would we?
There is a book called The Secret Life of Plants. In this book, there are a series of experiments performed on plants to see if there is any kind of activity that can be measured in the plants as a response to a given situation. I will talk about one of them.
There was the experimenter(s), five volunteers, and two plants. One of the volunteers was selected in such a way that no one else knew who was chosen. The two plants were left in a room that anyone could get into. The selected volunteer was then to go into this room and take one of the plants and chop it up into pieces with a knife in front of the second plant. After this was done, the second plant is hooked up to a polygraph. Now before you laugh, I want to explain how a polygraph works. A polygraph does not read your mind. A polygraph measures electrical impulses in the body. It would be just as effective in reading similar impulses in the plant as in a human. So they hooked the plant up to a polygraph, and marched the five volunteers in front of the plant. The plant had similar reactions to four of the volunteers, but on one of them, it reacted quite differently, quite a remarkable reaction. This vounteer was then asked if (s)he was the one that murdered the first plant, and it turned out that s(he) was the one. There were other experiments also, but this is all that I am going to discuss here.
Let me know what you think on this matter.