Of course, the author is Tang Xianzu.
The original text is: for you, it is as beautiful as a flower, passing like water, and for children, it is a leisurely search. You feel sorry for yourself.
What Liu Mengmei said is: (I) searched here for no reason, but (I don't know) you, a beautiful and fleeting couple (referring to fleeting youth, which actually means "young and beautiful"), are alone in a quiet boudoir.
But the first sentence together is definitely not appropriate. Because it doesn't fit the story.
The origin of these two small conjectures is Li Bai's poem "The Long March": "My hair has not covered my forehead, I am picking flowers and paying by my door. When you, my lover, ride a bamboo horse, trotting around and throwing your childhood, we live together in a long dry lane nearby, and we are both young and happy."
As for where these two sentences come together. Forgive my ignorance, but I really don't know.