Geo, so sorry for your loss. An animal loss is really tough. I think it's because the relationship is all about the physical presence and nothing more. I can be away from a person for quite a while and still connect with him/her by phone, e-mail, letters. I'm O.K with this because we can still be communicating. It's not the same as being around them, but it helps. I might go a year without seeing certain loved ones without feeling too badly about it. On the other hand, if I'm away from my pets for a couple of weeks, it really sucks. I can't just call them and see how they're doing. They have to be there, I have to see them, hear them, pet them, hug them, it's an absolute must.
These are just my thoughts on why an animal loss can rival that of a person. Another thing is the sense of innocence. I really doubt that there is any evil in the animal world. They don't have ill intent. It's usually been unconditional love.