First Act of Betrayal

Not that I like to me dramatic or anything - anyone who reads my blog, will know that I am never dramatic - but I have just paid the deposit to transport two cats to England and I feel like Judas. I feel as though I have blood on my hands.
Because, you see, there is our third cat, my cat, my baby, who will be staying behind.
Simba is my three legged, 17 year old, depressed Tonkinese cat, who wakes me up several times a night for cuddles, to go outside, to come in again, to retch filthy smelling breath into my face, or just because he feels like it. He is the main reason I have been a zombified mess these last many years. He has cost us an arm and a leg in vets bills, what with tick bites, leg amputation, a magpie attack, cystitis and, now, renal failure that he should have died from two years ago. I have spent many, many hours nursing him, force feeding him, comforting him when his brother died, massaging him when he suffered from phantom pains, cleaning his blood and gunge from the floor and our bed. He poos in the bath, wees on our bathmat, pulls out his fur and leaves it all over the house. 
And, for some, unexplainable reason, I love him. I love his little grouchy brown face, his hop-a-long gait, his skinny, patch-haired body, his knobbly sharp spine.
However, the Gods, (otherwise known as Rupert and the vet) have declared that is is really not worth spending a fortune transporting a cat in his state to England - in fact, the flight alone could be the end of him. So I am farming him out, as though he were nothing but a pet that is no longer wanted. 
Why is it that we feel like this about animals? What biological/evolutionary trait, is it, that makes us humans love something which does not bring us any sort of advantage, that, in fact, cripples us with exhaustion and stress?
I am just hoping that he doesn't feel anything like as strongly for me, that he will forget me as soon as he gets his first piece of cheese from my friend Julia, his soon-to-be-new-Mum.