It's really easy for dogs and cats and horses (I mention because those are what I know) to 'read' emotions. Like human babies, they are tuned into Source, and their connection with 'all that is' , is 'pure'. It doesn't have everyone trying to 'train it out of them. So of course, they never told themselves they could not. We call it instinct, but that is what it really is.
It's one of the reasons I love animals so much. They naturally are who they are, and I so appreciate them for feeling the energy when I'm not feeling well, as well as feeling it when I'm sad or whatever.
I love how when I'm sick they don't 'hound' me, sorry, I could not resist, to go outside, etc. They just know. What I don't really like is how they can get depressed along with me. That bothers me, and I'm glad when they don't stay that way also.
Animals 'feel the energy' because they have not put up barriers to their 'knowing' it like we have. It is just there, like 'reading the newspaper', so to speak.
I have even seen people become more aware of energy by just paying attention to their animals. Everyone nowadays pretty much knows that dogs can feel fear. That is just one example, but we can consciously know it too. I love how I have seen this 'wake people up' spiritually.