Why did Ash give up Charizard?

Why did Ash give up Charizard? 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.