Well, the whole Universe is made up of energy, in varying frequencies of vibration, including you, me and your animals. When we realize we are energy beings, it is not hard to understand that energy has no barrier, boundary nor separation.

We can put a wall between a space and call it two rooms, but we all know when our kitchen smells of apple pie, that smell permeates the whole house. When it comes to energy, the walls give us only the illusion of separation.

Thoughts, feelings and even physical forms are different manifestations of energy, one denser than the previous. So in reality, one can tune into someone else's thoughts, feelings and body to sense/feel how they are doing or being.

Animal communication and pet psychic mediumship are based on the same principle. We all have these abilities. I personally always ask for permission, on a soul level, before tuning in for information.