I like to belive that when Clow introduced Yuuko to his guardians Yue became super jealous and started glaring at her unstopable and furiosly. And being the oblivius morons they are Clow decided that Yue was starting to have a crush in her (projecting is also part of Clow Reed, great magician, line of thinking) while Kero was sure Yue was trying to challenge Yuuko to a "who blink first" competicion.
Yuuko notice cause she isn't stupid and her and Yue are sharing the two brain cells there and talked to Yue in her weird way of being really nice and they bond. When the day is over Yue is super frustrated cause he now understands what Clow sees in Yuuko and can't hate her for her and Clow mutual feelings.
Kero and Clow both act extra nice with him all week (Clow beliving Yuuko told Yue she didn't felt the same and Kero that Yue lost the glaring contest) and for years after that Yue will always belive that Clow gently talking about unrequired feelings and platonic love being as good as romantic love was because he finally noticed the guardian feelings and was trying to turn him down nicelly.
Years later, Eriol would look at this memory, understand what happend and laugth for a whole hour about how blind his other self was.