I can relate to the feeling of "not signing up for" enduring their illnesses/death. Not in a selfish way, but in sort of a "suspended ignorance" way. I wonder if there's a part of our being that shuts off that bit of forethought when taking in a(nother) pet. If we walked around every day with the full weight of knowledge/memory of what it's like to lose a pet, I'm not sure we'd ever get another.
So sad and yet in a way beautiful. All of us who have lost a pet can relate. We lost 2 cats - Boots and Ali - to kidney failure, and BJ to a heart and cancer problem. It's so very hard and we wish they could have been with us longer. But we treasure every moment we had together and they will always be in our hearts.