From the very start, BB’s dogged presence, bad attitude and jumping to conclusion notwithstanding, SS could not help but acknowledge her instinct to often zoom right into the crux of the matter for him, affirming or stimulating his thoughts. For instance, he had this notion that Hau Yee was murdered but BB cemented that notion into reality.
Also, BB has the ability to burrow into SS's conscience to challenge things he would rather ignore. Like SS's emotional desire to cover up Ka Seng's crime despite his conscience over the injustice done to Hau Yee. BB says harsh things to him that pretty up summed up what his righteous self must have had said to his conscience.
Later when he got to know her better, the acknowledgment of her instinct to connect with his inner thought turned to appreciation and wonder as in episode 20. "You've the knack to vocalize what's in my thoughts."
And also in epi 22 when he tells her that she is like a worm inside him knowing his inner thoughts. And that compliment warms BB for who wouldn't like to have one's own opinions so valued by the person one loves. And that scene is so precious that words can't adequately describe SS's gentle ribbing of BB's postulation of her ideas (incidentally reflecting SS's very own), and his indulgent/adoring look, appreciative eyes and his teasing smirk. Yes, Steven and Tavia just fed off each other acting so well.