this question has been nagging at me so i'm sorry if it's a topic which has been beaten to death already, i just haven't seen it!
so, here it is:
why are some subeta pets, well, pets, while others are clearly independent in their own right? it's similar to the questions that people might ask about settings like animal crossing, i.e. why can i gift my octopus villager a non-sentient and non-anthropomorphized octopus - but as a general rule, subeta pets can talk (when they're fed or red a book for instance), so sentience isn't a distinguishing factor. and then what about jim, for instance, who used to be a charlie but now is a human? as far as i know this is just a retcon situation, but what if it was something more?!
if there's actual lore answers to this, i'd love to know! and also, if not, i would be glad to hear some headcanons about this :D why do you think some pets choose to be pets to humans, and others remain independent, even living alongside humans as equals?
Captain
There is no explanation for Jim having been a Charlie. Euclid was a wyllop and the Subetapedia entry for Ian Asmodeus says
There is no explanation for how Euclid regained his human form, though.
As for everything else we've just got to suspend disbelief because the fictional world of Subeta doesn't make much sense. Any pet can have any job so a fish can dig graves in Shadowglen. Any pet can battle, too, even if they have no way of holding weapons - Blobs have no limbs.
Wasn't it mentioned somewhere that the spirits from Shengui Guo had something to do with it?
Perhaps they just got a good education
Lumi tree
Kind of a rough analogy, but that sounds feasible to me.
As for why some pets in Subeta are pets and some are sentient beings - I think of Subeta as being its own ecosystem. In our world, we have a very binary and exclusive system. Only one species has evolved into sentience, and the rest have not. There are, of course, varying levels of intelligence in the animal kingdom and a lot of debate about that, but in general most people agree that animals are animals, and what separates humans from animals is the superior evolution.
("Superior" meaning next step and also not an indicator of intelligence.)
Subeta, on the other hand, is more fluid in terms of sentience and non-sentience. You have the Official Subetapedia that explains which species are which, but I think this can be looked at more of as a guide. You also have how users treat their pets, and some people treat them more like pets while others treat them more like sentient characters (and then there's all the tributes, memorials, guides, encyclopedias, etc.). But that can be explained by a non-binary evolution, an ecosystem in which some parts of a species evolved and some didn't. Like how all dogs are dogs, but imagine a world where German Shepherds were sentient but Goldendoodles were not.
You also can't equate "pets" to "animals" in Subeta like you can on Earth. They aren't really the same.
So anyway, that's my long, rambly opinion.
There's also the trite answer: it's magical. And if Goofy and Pluto can exist together in the same world, why not in Subeta?
Heck, I even have a pet whose friends are all sentient vegetables because they were planted in special soil, but there are also non-sentient vegetables of the same kind in that world.
I get round the 'doesn't make sense' aspect by thinking of Subeta as being like Disney films, cartoons and children's story books where animals do human things.