When I was 5 years old, I promised the neighbor's girl I'd marry her and become King of Japan. 10 years later, she wonders why I haven't done both of those things yet.
It's a common thing that pops up in stories with a harem growing up: you have your Childhood Friend character who somehow remembers when you said you'd marry her and you were both five years old. Somehow that one utterance you said before you understood what "marriage" truly meant is taken as an ironclad oath when she coincidentally runs into you 10 or so years down the road, demanding that you marry her and uphold your promise. It is this promise that drives the romantic candidate, even though the MC himself may not remember what he said or why he said it at the time. Maybe the girl is desperate. Maybe her brain runs on one track and this the only thing fueling it as she's grown up through the years. Or maybe the writer just can't imagine why childhood friends don't always stay together, or if they do why they wouldn't lead to romance.
Whatever the case, some childhood promises are considered sacred by characters, others laugh at them or use them for parody. They still surface as motivation for some characters who knew each other.
Childhood promise may not always involve harems or romance, like Ash Ketchum's constant proclamations of being a Pokemon Master or Luffy's wanting to be "King of the Pirates" at a young age.