Why did Charizard leave Ash?

Why did Charizard leave Ash? Ash Ketchum decided to leave his Charizard in the Charicific Valley because he thought it would be better for him to stay there and train. He did not actually release him as he did with some other Pokémon, he just allowed him to be with his own species.

Ash Ketchum decided to leave his Charizard in the Charicific Valley because he thought it would be better for him to stay there and train. He did not actually release him as he did with some other Pokémon, he just allowed him to be with his own species.