Master Liu felt guilty about this, thinking that she was not only unable to help her senior, but also had dragged him down.
Therefore, she took the initiative to enter a deep sleep, which could minimize the burden on Yu Xian.
Yu Xian was deeply moved by this and agreed without hesitation.
How could he refuse Master Liu's kind intentions?
Furthermore, Master Liu's slumber was actually beneficial and harmless to him, especially since fewer people knew about the things he was secretly doing.
Just look at how Thousand Beasts True Master was almost scared out of his wits.
Master Liu, being awake and unable to be apart from him for a long time, would have tarnished his glorious image.
Later, he focused on his career and neglected Yulan and the others, much less remembered that Master Liu was still in his sea of consciousness.
Afterwards, he broke through to the Nascent Soul stage, which was a time when he could have awakened Master Liu.