As herd animals, horses have evolved to become keenly aware of their surroundings and those in it. You can hide what you feel from others—and even from yourself—but the tiny cues you give off can’t be hidden from horses. Happy, sad, angry, playful, shy, aggressive...a horse knows. And immediately reacts.

Also, like all animals, horses don’t care:

Horses want safety, comfort and fun...and they keep all secrets that you whisper to them.