'Replace' is the wrong word!
Although, like Penny I have an adorable little fluffy dog too, my black cocker was irreplaceable! I even burst into tears in PAH, and got hugged by a complete stranger, who cried too. I couldn't look at any cocker for a year, till I met a couple with gorgeous chocolate workers. I asked if I could stroke one of them, and received the wonderful , silky cocker kiss, and I held back the tears and felt comforted. My remaining dog was turning into a spoilt baby, unable to be left on her own, so I decided to get another rescue. Someone had beaten me to the cocker, so I was attracted to a scruffy, grinning terrier, straight out of a 'William' book, like his dog Jumble. OH was brought up on William books, and has bonded much better with him, than any of my spaniels and lab, which he thought were wimpy mummy's dogs!
Maybe it's because Bob didn't get to to have his full length of life. You feel hurt for them. It's the same with people. I'll be sad when my mother in law , aged 96, dies, but not broken hearted. ( But wouldnt I replace her)!!