Whether they pose a threat to people or are easily harmed by people, whether they have very specific nutrition requirements, whether they are rare, whether humans can provide analogous environments, and of course convention. there are good reasons why many animals shouldn't be kept as pets. The idea that a safe life as a pet is somehow superior to wilder life has no scientific basis though, it is based on our assumptions on what constitutes a good life. H Having said that, I do think many pets have pretty darn good lives, if quite tedious for a fair amount of their day.
