Why does every couple have to have met in their childhood ?

I'm recently reading Open the door ( an esperguideverse ) The story isn't inherently outstanding since the seme comes off as toxic but out of the blue mc gets a flashback of a child who saved his life but probably is dead now. I have nothing against properly established stories of reunion where the MCs actually had something to do with each other but more often that not it's done lazily by authors just to have something like fate / they were meant to be together / oooh they knew each other as kids and is sloppily done without any prior context or not elaborated. It's really annoying me.

There's another trope where adult MCs are weirdly portrayed with child / baby mls ( even in art ) or one of them turning into a child.


Some examples:- from stories I don't remember
Mc remembering ml bought him a can of coke ( this is probably from omega complex or sm )
Mc buying icecream for ml in their childhood
Mc saving ml from dying
Mc giving candy to ml in their childhood
Mc passing by ml when they were kids
( My point is there is absolutely no significance to the relationship whether they knew each other in their childhood or not )