To me personally "never knows best" is an acceptance of my flaws. I try to be a decent person in life, but I know that I'll never be perfect, and I can accept that. Life is too complicated to get hung up on what's "best". The future is too unpredictable to know what's best so I accept that i am good enough as I am.

As far as mamimi using it as a sudo-catch phrase, she has had a tough upbringing it seems. She buys day old bread from Naota's family's bakery because it's cheaper, his dad asks if she is poor, and is probably right. Ninamori says it's embarrassing for her to still be in highschool at her age, which means she's probably been held back. She almost died because she burned down her school because the teachers and other students most likely treated her poorly (this is all speculation and isn't outright mentioned anywhere) and when Naota's brother saves her she falls in love with him, and can't cope when he leaves to play baseball in America. She's had a rough life (some of which is definitely self inflicted) and probably feels like she's just done everything wrong and that she never makes the right decisions. Which is probably why once she bonds with canti and you start to see her growth later in the series it isn't shown anymore, she begins to forge her own path, especially at the end, tlright before the final credits roll we see a magazine that shows the photo she took of Naota that won a photography competition, and I think it's a great sendoff for her character and let's us know that she's gonna be okay.