We as a fandom do not talk about this scene nearly as much as we should
Like, I had already headcanoned that Nya's independent personality was a result of her upbringing, but the fact that this scene officially confirms it is just *chefs kiss*.
What I find so interesting about Kai and Nya is that their childhood trauma has very clearly effected personalities and overall characterizations. Both of them have lone wolf tendencies (albeit more so in earlier seasons), often tending to go their own way and handle their problems on their own instead of relying on the team. They both have anger issues and short tempers, struggling to process their emotions in a healthy way. And neither of them are comfortable with weakness and vulnerability, and tend to lash out when they feel cornered.
There's also the fact that, due to growing up as two kids forced to raise themselves and learn survive with limited assistance from their community, both of them are understandably afraid of feeling weak and vulnerable. What I find to be perhaps the most interesting part of all this is that despite sharing a similar fear, both of them have vastly different coping mechanisms for dealing with it.
For Kai, his fear of powerlessness manifests in a desire for more power - a desire to be the green ninja, a desire to protect his friends, a desire to be strong and heroic, and of course a difficulty with accepting the situation when he loses his powers. He has pretty seamlessly integrated his power into his persona as well, frequently referencing his abilities and praising his status as the master of fire arguably more so than any other character. He's quickly turned his power into a huge part of his identity. Every time he loses his powers, he tends to go into a bit of a depressive episode - in season 5 he developed a fear of water, and in season 11 he became so upset that he nearly quit being a ninja altogether. When he failed to save the Never Realm villagers from Boreal, he blamed himself for it by lamenting that he wasn't powerful enough to save them.
You get the idea. Point is, Kai doesn't like feeling weak, he doesn't like vulnerability, he doesn't like being the weak link or a liability holding everyone back. Personally, I would point to that insecurity as being the primary driving force for his determination to become the green ninja, but that's definitely up for debate.
And as for Nya, this fear has manifested as her being a fiercely independent perfectionist, equating mistakes and failure with weakness, and fearing failure to such an extreme extent that she either gives up anything she's not immediately good at, or completely ignores her problems and pretends everything is fine. Like in season 5 when Wu was training her to become the master of water. "I'm not weak, this bucket is weak! I'm good at everything else, why do I have to be good at filling buckets?!" Or her struggles to get along with her mother all throughout Seabound, knowing full well that she can't blame her mom for leaving but still not quite ready to have her mom in her life again. She also isn't good at asking for help either, often preferring to handle things on her own even at her own detriment, and often she has a hard time admitting when she's wrong as well.
Whenever traumatic things happen, she just steamrolls right past it, refusing to acknowledge her own grief for very long. At Zane's funeral the first thing she said was, "So...what happens after this?" In the face of Maya's repeated attempts to reconnect and talk about Nya's childhood trauma, Nya would often shrug her off or storm away or change the subject.
Maya tried to explain her reasons once - tried to explain that sacrificing yourself comes with immense collateral damage, often hurting the same people you're trying to protect, and says "That's just the way the cookie crumbles." But Nya can't bring herself to address what Maya is really getting at, instead swimming away and saying "I don't like crumbly cookies."
That's why her final words are so important to me. "Tell my mother I understand now. That's the way the cookie crumbles." Nya has finally come to terms with and accepted her mother's decision, accepted that not all solutions have perfect answers, and has finally confronted her own baggage once and for all. She understands why her mother left, she forgives her, and above all she, the team perfectionist, is making a decision that isn't perfect. A decision that will leave a lot of collateral damage, and will cause serious trauma to her loved ones. She knows that by leaving her friends behind and becoming one with the sea, she's hurting them the same way her mother's absence hurt her - but she does it anyway because it's the only way to save their lives.
And as she looks back to say goodbye to Jay one last time, she cries. She mouths the words "I love you" - something she'd never said out loud before. She lets herself be truly raw and vulnerable. Even during her death in Skybound, she was still trying to keep her up her strong and defensive facade, saying things like "I never wanted to be part of your boys' club anyway". Even on her deathbed, in that moment she can't bring herself to let down her walls all the way. But here, several seasons later, she has finally let herself be vulnerable enough to cry, to say goodbye, to say "I love you."
Nya may not be my favorite ninja, but she does have my favorite character arc. Her going from samurai to ninja was a controversial decision, but I really do think it was a good choice in the long run because it almost feels like it's symbolizing her growth - going from a lone wolf who insists on doing things by herself, to being part of a team and relying on her friends just as much as they rely on her.
Just...I love Nya so much. She needs a hug.
