Slight re-framing of the question, since I don't always make the same choice:

"What factors influence the life-and-death Choice(s) you make ... about your pet?"

Statement of Principle: I love my fellow critters (as a rule, tho I make conscious exceptions) - the ones in my family, the ones I meet casually, the 'wild' ones in nature. I enjoy eating (some of) them. Generally, I wish them to enjoy life as I do. I do not wish them to suffer. That said:

Quality of Life : If there is suffering, there is an implicit choice. The decision - not to be cruel - is something of a cost/benefit analysis. When alive, my parents assured me that they loved life. They also did not wish to be a burden. Makes sense.

Survivability: Some stuff - like 'old age' - is inoperable. My recent critters-of-choice are cats ... and rats.

The rats (introduced by my son) were a delightful surprise: They are social, very intelligent, "fast learners", soft 'n cuddly, and remarkably affectionate. They just don't live very long (3 years is a good run) and are very susceptible to tumors and respiratory problems. They're also quite small and few veterinarians have experience in care or invasive procedures. There usually isn't a lot that can be done.

The Value Proposition: Let's face it - Some of our fellow critters aren't highly valued. The folks at the local PetCo were delighted when my son and I selected a new rat to join the family (Rats are social animals and are happiest when they have company). The PetCo folks both appreciated that rats are pretty decent companion animals - and knew that most of them would be sold as 'feeders' for other critters.

There are no rats in my house these days. PetCo had - I am told - an outbreak of 'rat scratch fever' in one of their stores and decided not to carry rats any longer. Just business. Nothing personal.