Maybe it's an introvert thing, but I seem to develop a stronger bond with pets than with humans. My cat of 13 (human) years died alone, I grieved. Still years later, ever so often I would cry at night saying, sorry, that I wish I could hold you as you laid dying. She was a very devoted cat and saw me through my adolescent years, lay with me when I cried. I confess that I had mourned for her more than my own grandma, because of how deeply that bond was. She was with me every day 24/7 for 13 years. It's hard for me to let go of that bond.
And that type of bond is right outside next door, and I can't have it anymore. I just see Sadie ever so often outside. I don't know how to let go of something that is not actually gone.