The Biophilia Hypothesis by Edward Wilson posits that our connection to animals is rooted in the history of our species' survival. We were once very reliant upon other animals to alert us to danger and changes in our environment. Seeing a peaceful animal allowed us to feel peaceful in turn, allowed us to feel safe, and our nervous systems to calm.
In moments of fear, our bodies tense, our muscles contract, our cortisol, adrenaline, and other stress hormones flow freely. We also experience a corresponding rigidity of thought and emotion, one in which we perceive things as polarized, safe or unsafe, black or white. Nuance and flexibility are lost in our need to prioritize self-preservation.
By contrast, when we feel safe, there's an emotional, mental, and physical relaxation that occurs; one which creates an ideal environment for creativity, change, and healing. A therapy dog helps us connect to that state; his calm or joy serves as a visceral reminder that in the moment, and in the therapy room, we are safe, and we can relax.