With Zhu Rong's vast experience and the information he has heard, it's clear to him that Chen Sixuan harbors feelings for Ji Dong. Considering Chen Sixuan's status, strength, and beauty that rivals the Flame, Zhu Rong definitely hopes that a relationship can blossom between her and Ji Dong. He understands his apprentice better than anyone; Ji Dong is as obstinate as he himself is, once he commits to something, he's unlikely to change his mind. Unless he falls in love with another woman, the grief caused by Flame's death will never fade from his heart. Naturally, Zhu Rong wouldn't like to see such a situation, and thus, this arrangement was the obvious solution.
Upon hearing the news, Chen Sixuan was overjoyed. She hardly slept last night as Ji Dong dominated her thoughts, a combination of pain and pleasure that was incredibly exhilarating.