自分が月の都の人間であり、満月の夜に月へ帰らなければならないから

From the spring of a certain year, Kaguya-hime began to sink into deep grief whenever she looked at the moon. When the old man asked her why, the princess said through her tears, 'I am not of this world; I came from the Capital of the Moon. On the upcoming full moon night of August 15th, an escort will arrive, and I must return to the moon.'