boogie, have you ever lost a pet that you were extremely close to?
i don't consider my pets my best friends -- i consider them family members. they're more than just friends or even companions to me, they're the spirit of my home. they add warmth and happiness to my life in a different way than other people or even "friends" can. that doesn't make them any more or less important than people, they're just different.
there's a special kind of contentment that is absent when i don't have pets at home. it is that feeling that no matter how bad my day is, no matter how unlucky i am or how horrible i feel, my cats will be there when i come home. they will be purring, they will be happy. they will curl up next to me and just be there.
they don't want me to talk about my problems with them. they don't want me to cry on their shoulder until it's all better. they don't want me to tell them what happened. best of all, they don't have better things to do. they always have time for me. that's something even boogie's definition of a "best friend" can't give you. and that's something i appreciate immensely, from any pet, be it dog or cat... the fact that this animal, this being that is so completely different from me, can sense my sorrow and comes to me. they perhaps don't even realize that they're being comforting. they are just there, and they ask absolutely nothing in return.
that's a connection that you can't have with a person. you just can't. with people, it is always give and take. with animals, it is simply give. i, personally, see it as completely justifiable to describe a relationship like that as "best friend."
edit: i might post this at gaf, to try and comfort the OP over there. i understand what he's going through.