Gorgeous! I just found this years Christmas presents for everyone on my list and a few grumps that need enlightenment :-)
I think the reason we 'seem' feel this so-called 'deep' feeling when looking into an animals eyes ie dog/cat etc, is actually based upon the natural reaction of the animal we are looking at that holds our gaze into their eyes. This is because an animal ie lets say a dog, will instinctively use most (if not all) of it's senses when we look into their eyes, has a why to identify who we are, what our sent is giving off, our body language at this time and facial expression, just to establish 'their next move. So in short, it is they who are looking deeply into our eyes to measure us up. My beloved late Labradoodle, Max (who sadly passed with out-of-the-blue sudden diagnosis of leukemia in 2020) would do this all the time when you tried to trick him, or tell him to do something new, which was not in his normal trained commands (he would completely hold eye contact, watching and listening to you - trying to work out what you were commanding him to do. He would stare you out instinctively, while he was trying to 'establish' the moment. He would also keep perfectly still and hold your gaze if you put a mobile phone or camera in front of him and said the word "photo" (he would have been an ideal subject for your book!) and he would not move until he heard the click!. But yes, I think it is the former, when we look into an animals eyes. Finally, I think your portraits and your book are wonderful, but even more, that you are fortunately privileged to be in the presence of such wonderful animals.
@EKQMarx