It's a matter of the trainer getting attention for what the animal is doing and they enjoy having a relationship with the animals. I trained my dog to do all sorts of neat things and he loves to do them. It gives him a sense of self esteem, I think, but he's a dog. He is never happier than when he's rolling over or touching his nose with his paw. He can even sneeze. He is a dog, though, nothing exotic or wild. My dog enjoys the interaction but my cats are totally not into it. It depends entirely on the animal and some of them are treated harshly for not training so well. I don't treat my pets harshly because they don't want to do something except if it's destructive or dangerous, then I make them cease whatever it is, of course.
My dog loves his treats and will do all kinds of cool things to get them but it's just between me and my pet, not to get attention from anyone, just the interaction between us is therapeutic for the dog.