I'm gonna go out on a limb and say this: For the sake of having a spiritual-connection with the show (let's be real here, bending isn't possible in the real world), It's nice to think that a bender get's his/her ability by being born with spiritual and mental traits regarding that element. It doesn't completely fit, mind you. makes you wonder why katara's mom wasn't ACTUALLY a water-bender, because by the implied qualities of her throughout the show, she had the good-hearted, helping and bravery of a water-bender. Maybe nobody has ever tried. maybe if your not born a bender, you can meditate and change yourself enough to learn it (or beg the avatar to give you the ability) the problem with this topic is that only the deep thinkers really get here, and sadly, the show NEVER went into detail about it. sure it had those twins and Mako and Bolin, but it's never really explained thoroughly. If your reading about this, you were probably one of those people who were wondering about non-bender oppression and bloodbending before you even finished the first season of TLA. I'm just saying that if everyone here wants to have a bit of fantasy in there lives, it would be smart for the writers to one day say that bending is quality of heart and can be attained through mental concentration and determination, so that way we can all feel a little-bit less hopeless