Some animals are your heart. Not your love or your baby or a friend, but Your Heart.
You know it from the moment you meet them. It doesn't matter if they are 2 days or 10 years old when you do.
It's not that you can't imagine life without them, it's that they are your life, and you theirs.
This phenomenon occurs more often and more obviously with dogs, I think. But in my case, it was a cat.
When he left me, my Heart left me. I will be recovering from the loss for the rest of my life.