This stems for a widespread, yet sourceless, anectode about english teacher and writer Natsume Soseki. Allegedly he once corrected one of his students, who tranlated the english 'I love you' word for word in japanese, on the grounds that japanese people, by culture, are not that direct, especially in romance. He then stated that commenting on the moon being pretty was more than enough to convey one's feelings for the loved one.
This stems from an antiquated, yet extremely romanticized notion that men shouldn't express their feelings openly. It was more appropriate to use roundabout ways, that combined with the situation (perhaps a moonlit walk with a member of the opposite sex) would hint at unspoken feelings of affection, subtelties that remained for the other one to pick up.
Nowadays this anecdote is so well known that it can be found in many mainstream medias, such as dramas, literature and, in this case, manga. Too bad Itsuki doesn't seem to know.