Don't forget that Aang had been a monk most of his life. Tenzin is very similar to him, except he had siblings that obviously picked on him. Korra has been exactly like a brash, temperamental toddler since we first saw her as a brash, temperamental toddler. She's a waterbender-Avatar who primarily uses firebending...
That's her personality, and why the spiritual side of things has always been so alien to her. She's about the least emotionally placid bender we've ever seen. She's on the level of early-Zuuko and Azula really.
Agree with Yuki regarding the female twin. Mai did deadpan so much better.