Fairly simple concept, I'm looking for a story where a boy and girl are childhood friends, but one day the girl sort of changes, maybe drops some of her hobbies that she loved to play with the boy, and begins to distance herself from the boy, only for them to wind up once more involved with one another years later. Two examples of just what I'm looking for are Dokyuu Hentai HxEros and Shishunki no Iron Maiden.

To elaborate, in HxEros

Kirara and Retto were always together as children, and Kirara was rather aggressive in pursuing him as more than just friends, until one day a monster attacked. The monster consumes "H-energy", an energy connected to lust and other desires. It drained Kirara, but Kirara was completely fine while the monster was overloaded to the point that it exploded, though only after ranting at her about how it'd drained a hundred times as much energy as any normal person could ever have, and wondering at just what could make her into such a raging pervert. After that she refused to have anything to do with any boys, especially Retto, and blocked out a number of her memories. Once in high school it becomes clear that the monsters are still around, and Retto is fighting them, and she ends up joining the team.

And in Iron Maiden

Tsukiko used to be best friends with Haruki, always jumping across from her window to his and playing TCGs and other stuff, until one day a seminar was held for all the girls. Almost immediately afterward she gave him all her cards saying she was done playing them, started calling him by his last name, and refused to hang out with him. When they reach the eighth grade he's still rather bitter that she'd changed so much, only to wind up forced to get closer once more when he discovers her secret, a secret related to that seminar that she needs to keep from absolutely everyone as the government would want to catch her and take her away to do experiments if they knew (one girl at the seminar went to the doctor who presented it and disappeared the next day).