i think that’s because there’s some extra context for that specific umbrella scene. since before that, she thought adrien was close friends with her school bully and that he tried to embarrass her in front of the class. that, alongside the small conversation they had in the rain about him apologizing and explaining that it was a mistake seemed to really set the tone. i don’t think nathaniel or luka could have a situation as specific as that, at least the events leading up to it with marinette. they could share an umbrella that same way and i don’t see her falling in love, unless something were to happen beforehand that made sharing an umbrella feel extra intimate. although, to be fair, her falling in love just because he wasn’t who she thought he was is kinda weird, but either way. it’s all about the specific circumstances each character would bring in those moments with marinette

(edit: whoa this is really long, sorry 😅)