This is fan fiction, written by a fan. I make no money from being a fan. The owners make money from my being a fan.
Theories
One of Zuko's tutors had told him that boys fell in love with women who reminded them of their mothers. Odd. The older he became, the less sense this made, and so the more he thought about it. Years later, Zuko knew he could never marry a woman like his mother. Ursa's memory was a sacred, untouchable thing. Loving Mai was nothing like that.
The Mother theory was presented to the prince around the same time Mai was introduced formally at court. Azula had wanted to meet the students from the most influential families before starting her courses at the Royal Fire Academy for Girls. Five girls of good-breeding and fine manners came to the palace, and three of them faded immediately into the background.
Two had been able to resist his sister's supremacy. One, through an effervescent will unlike any seen before in the hallowed halls of royalty. Even the servants most skilled in dealing with children foamed at the mouth by the time Ty Lee left. Mai, however, let the princess's malice roll over and around her without allowing it to touch a single super-straight hair. She was slightly terrifying in that way, and at his young age, he tried to spend as little time dwelling on it as he could.
It was many years before Zuko understood what she represented in his life. And by that time, his anger and bitterness overwhelmed all but the memory of red silk and a biting wit.