Why do some Pokémon act as pets, while others act as people?

I have always confused the pokémon minds. Some Pokémon seem to have the behavior of an average domestic mascot, while most legendary Pokémon have the ability to speak human language, often telepathically. And although one could assume that a legendary as Arceus would be more complex than his typical Caterpie, things are not as simple. Not all Pokémon seem to have the same level of consciousness, even between the same species, and some common Pokémon seem to be less anthropomorphic than legendary. The more I think about it, the more confusing it becomes.

Team Rocket Meowth is the first example in which most people will think because it is one of the strangest ones. Meowth shares many features with the average team of Team Rocket. You can speak fluently, feel empathy and plan as well as Jesse and James, so it is worthwhile. In the episode of season 2 Go West Young Meowth, we learned that Meowth was just a normal Pokémon that ran with a gang from Meowth's street in Tinseltown, the Pokémon version of Hollywood. He and the companions of him meowth behaved as vagabonds, stealing food and rummaging in the garbage containers, until one day he met a Meowth called Meowzie and fell in love. Mewozie rejected him, so he taught himself to talk and act as a human to impress her. Meowth's transformation would seem to indicate that the only one ...