Maybe I live in a different part of the world, but I don't normally see/hear strangers or distant acquaintances consoling one another by kissing them on the cheek. That's a very vulnerable moment to be in, and I see it as pretty awkward if someone was to kiss an almost stranger on the cheek. I've consoled even good friends before, but never have I kissed them. To me, that involves much more familiarity with someone. Sure, in some areas of the world people kiss on the cheeks as a greeting, but this is definitely not that as it's neither a greeting nor a farewell kiss.
I do live in country where kiss on the cheek is an uber common greeting.
If you as a guy were to give a girl you have a limited familiarity a happy birthday, kiss on the cheek is definitely the way to go(and vice versa), man to man is a hug and some back taps btw
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Where i live it's very common to stop and say "hey are you ok?" if you see someone you don't even know in distress, hugging them... if in addition to that you give them a happy birthday, 100% you kiss their cheek(both cheeks actually).
IIRC (been a while)...other than the flashback...neither do we see any relevent backstory regarding Erynn and the MC either....other than that event...
Why is one 'not being seen' so much more important than the other to some people?
Don't make it too easy... this smoking gun is filled with blanks
Just off the top of my head... Things that get mentioned/shown, Erynn talks about she remembers fondly teaching him math(and how this led to her career choice), using him as a barbie doll(implied to be more than once), giving him piggyback rides, how much she loved tickling him(implied to be so constant, that he's now insensitive to it), Erynn sleeping in his bedroom... and even allowing the kid to touch her boobs.
That's the level of history and memories that they share with each other and remember fondly.
Those are all memories explicitly stated, off course even in the original flashback(the one from the plane) it's shown, in a more general manner, that they have been friends and playing with each other since before Erynn got to her teens and met Ethan.
This is why they remember each other, why they have a connection, this is why they matter to each other...
Why in the time they spent apart they still thought about each other(MC making a song about Erynn even), all explicitly, factually stated.