Watching human minds work

I am a great people watcher. I love to people watch while sipping on a hot cup of coffee outside the local Starbucks. What is going on when they are frowning? Is it a true frown or are they simply concentrating? What about when they are smiling? Are they happy or thinking about a happy occasion? Is the smile a clown smile? Happy on the outside but sad on the inside?
I also do this when I am watching people adopt an animal. Why do they chose one over the other? What is it about the animal that makes them fall in love?
Is it the look, or the sad eyes? Is it a long-haired dog or short? Is it a big dog or small? Are they timid or rambunctious. A lap dog or a ball fanatic? In most cases you can watch their minds at work, weighing everything about the dog from size to coat, to personality.
This all happens within a few minutes. Even when you think you know what they are thinking, you don't.
I have seen people come into the shelter looking for a small to medium dog with short hair and leave with a large dog and long hair. I have also seen the opposite. What is it about a particular dog which captures our minds and changes our decisions? The mysteries of human minds continue to facinate me.