i get so much mileage out of this story...
and from an animal behavior/science point of view, yes, you can pretty much know when a dog is happy. can you know what a happy dog feels like? do you know what a happy anyotherperson feels like? empathy - you have evolved the ability to do so, or at least convince yourself of that and make it work. and no one can tell the difference.
Zhuangzi and Huizi were strolling on a bridge over the River Hao, when the former observed, “See how the minnows dart between the rocks! Such is the happiness of fishes.”
“You not being a fish,” said Huizi, “how can you possibly know what makes fish happy?”
“And you not being I,” said Zhuangzi, “how can you know that I don’t know what makes fish happy?”
“If I, not being you, cannot know what you know,” replied Huizi, “does it not follow from that very fact that you, not being a fish, cannot know what makes fish happy?”
“Let us go back,” said Zhuangzi, “to your original question. You asked me how I knew what makes fish happy. The very fact you asked shows that you knew I knew—as I did know, from my own feelings on this bridge.”
and from an animal behavior/science point of view, yes, you can pretty much know when a dog is happy. can you know what a happy dog feels like? do you know what a happy anyotherperson feels like? empathy - you have evolved the ability to do so, or at least convince yourself of that and make it work. and no one can tell the difference.