Han Yu didn't just want to let Liu Xiu know the importance of Su Wenyue, to prevent him from acting rashly, and from making any unfavorable moves toward Su Wenyue that could cause irreparable rifts between them. It was also a declaration of himself, letting Liu Xiu know he was a man with little ambition.
Han Yu was never shortsighted; he wouldn't lose his ability to think just because of the slight kindness and apparent trust from Liu Xiu. No matter how much trust Liu Xiu seemed to show on the surface, what he really thought was hard to say. Even if he let emotion triumph over reason for a moment due to gratitude, as time went by, it was inevitable that he would start to think too much, especially given the transformation Liu Xiu had undergone, shedding his previous soft-heartedness. While this may be good, it was a double-edged sword for his subordinates. They'd benefit from using it well to avoid harming themselves, so all the more they had to be careful in their actions.