Actually, Situ Xin wasn't being blindly optimistic about trying to set up Liu Yuxiang with Lu Nini simply because he liked her character; as an observer, Situ Xin could see things more clearly. Despite Liu Yuxiang's verbal complaints and the contempt shown on his face toward Lu Nini, it was all just a facade. If Liu Yuxiang truly disliked Lu Nini as much as he said and showed, she certainly wouldn't have appeared in his room, and Situ Xin wouldn't have met her.
In fact, Liu Yuxiang had not yet realized his true feelings. Moreover, Liu Yuxiang couldn't let go of his pride to admit that he also had a fondness for Lu Nini in his heart.
And at this time, Situ Xin didn't enlighten Liu Yuxiang, waiting for him to discover it on his own.