It always struck me as odd that Avatar Wan entered a permanent partnership with a female spirit, who vowed to stay with him forever and that not even death would do them part, and that every Avatar that we've seen to have followed him has ended up married to/dating a girl. Wan got Raava, Avatar Kuruk got what's-her-name-the-chick-who's-face-was-stolen-by-Koh, Avatar Roku ended up with Ta Min (I think that's her name?), Aang married Katara, and Korra ended the show with Asami in tow. Parallels, anybody?
My theory is that while most Avatars didn't remember Raava consciously, they still instinctively looked for people who were similar to her when choosing life partners.
Raava was fierce and bright and naïve when it came to humans. She did her duty, even when it meant fighting her brother for all of eternity. She was loyal and impulsive and fallible—she was Wan's brighter, older half but that didn't make her any wiser than him. The age before the advent of the Avatar was called the Era of Raava. To me that says that she was akin to the queen of the world before Wan came along.
Let's see Avatars living out their lives looking for this brilliant queen and finding—not her, not exactly, because Raava's light lives within them and she's never truly left them—but finding queens among men, blindingly brilliant women who stick to their guns but still apologize when they're wrong. Who get even when wronged but still bow their knee to give a helping hand to those who need it. Who fight back, who hit first, who make the best of a bad situation, who live their lives with dignity.
Maybe they have blinders on sometimes. Maybe they hurt more than they heal, do more harm than good. Maybe they never apologize for doing what they think is right, even when it's wrong. Maybe that's okay. Raava started out thinking Wan was scum—he was human, and he freed Vaatu without knowing anything about the situation. She learned better with time. Raava restrained her sibling for millennia, and when she and Wan defeated him, she sealed him away for ten thousand years. Indefinite incarceration without a trial is illegal. She never apologized, because at the end of the day, Vaatu wanted to hurt others and she was doing what she thought was right.
But she still made her relationship with Wan work, and at the end of his life, I'd like to think that she'd come to love him. And I'd like to think that Wan loved her too, and all his successors still felt an echo of that all-consuming love that inspired him—that inspired them—to create the Avatar cycle. And maybe these newer Avatars didn't know Raava, but they remembered enough of her to want and to want to love a girl who stood up for her beliefs. Who was a queen, if not in the most traditional sense. Who was brilliant and radiant and endearingly, frustratingly impulsive. Who lived her life with dignity and kindness and sense of self-sacrifice.
(Who threw herself headfirst into a war when she could have stepped back and done nothing to spare herself the pain. Raava made war with her sibling, but—Katara lost her village and her mother and fought when it was safer to hide. Ta Min let her husband go to do his duty, when staying behind broke her heart and saw her a widow. Asami spent her life being told that she couldn't fight, not as a non-bender, and picked up a weapon anyway. They stepped forth to fight monsters, stayed back to hold the fort against traitors, redefined the front line in their own way. They were queens of their own battlefields. It's only Raava who ruled over others.)
Let's see the Avatars falling in love with these women not as a betrayal of Wan's silent vows to Raava, but as an expression of their devotion—as the unaware loving the women Raava might have been, had she reincarnated separately as a human.
(And in the end, who's to say she didn't? Wan and Raava were intertwined together so closely that they could no longer be separated. That the powers that Raava held for Wan were used so freely by him and his reincarnations. That when he entered the Avatar state, his eyes glowed and he spoke with both their voices. But Wan's reincarnations couldn't hear Raava. They didn't know she was there.
Who's to say that when Wan's soul was torn out of his body and reincarnated, Raava wasn't taken out too? Who's to say that her power remained with Wan and her soul did as well? When Raava was torn from Korra, she was small and weak. Why couldn't this have been because all that was left to be torn from Korra was her power, and her soul had long left the Avatar, had been reincarnated into a human with only the barest hints of her former power?
Wan's reincarnations couldn't hear Raava. They didn't know she was there. But every one of them could and did fall in love with a radiant woman who was fierce and dignified and kind. Who was their brighter half, but still so fallible. Who made mistakes, but did their best, remade their own battlefields in their images.
One day, children will sit in schools and recite all the Avatars they can name. They will never mention these beautiful, impulsive, fierce queens each Avatar loved. History will not record their names. But that's okay. They never remembered Raava, either.)