Why exactly do you think pets have the ability to tell when you are upset?

For all the years I have had pets, and I’m talking cats and dogs, they have been able to tell I was upset. They become extra loving and stick close to your side. It always amazes me, it’s always nice to know that you always have someone that cares. Why do you guys think this is exactly? Or does anyone know if there has been any research done on this topic?