Been quite a while since I've read this, but if I remember right Cai Seng was never in love with Yunshi. She was only friends with him, and wanted pay him and Pinger back for being friends with her in their previous life.
There were only two persons in her life she loved like that: She loved her "sister" (forgot the name) from afar as a man - although I'm not sure how strong those feelings were and not just 'him' settling for someone unavailable. Then came Wei Tzu Qiu into the picture, the love of her life -as a woman*- with whose reincarnation she meets at the end.
*was just pointing out Cai Sengs (preferred) gender at the time. Honestly in Tzu Qui's case I don't think the gender mattered, they were in love with the other person (no breeding purposes involved), Cai Seng chose to be a female 'cause Tzu Qiu made her feel like/wanting to be a woman. Quite the obvious choice considering it's a shoujo manga (lol), but still.
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the ending is quite good.. it just.. how can the girl like yunshi, but then just switched to that fairy boy??