My theory is, that Ash adopted and kept it alive when it was just a little Hitokage (ahem* Charmander), When it evolved into Charmeleon and Charizard it got rebellious because it probarly throught Ash wasn't mature enough to handle a fully evolved Pokemon yet. Charizard and Ash's relationship grew when Ash worked extremely hard to defrost Charizard in "Charizard Chills" You need to see that episode to know what I mean. It was an episode in Season 2 I forgot what Episode # it was. Charizard started to really respect Ash, he saved its life, and that care and concern actually helped them in the long run. I have a feeling Charizard will make a comeback sometime in Sinnoh or prehaps the Platinum version of the anime, but that's wishful thinking, through it is possible.