Didn't the Aide say that the duke must've fell in love at first sight to lady Valerie?
I think it was more like second sighting? He only seemed curious because she ran away during their first official meeting/dinner. Serenia was probably more on point that he started "falling" when he was looking at her holding his hand while crying in her sleep... though IRL that's creepy...
I think he just slowly falls for her because if her unique characters. The duke seems innocent in the "Love" area, looking at how he read all those romance novel for reference. So he choose a girl he thinks suitable for a fiancee, and then just started to really like her
Yeah, the development seems to be curiosity -> interest -> affection/attachment. At this point (the engagement ceremony), it seems to be something like a major crush instead of full-blown love, though if it keeps going in that direction, I have no doubt it would turn out that way.
The question in my mind is when did the duke even like Valerie?? Did he just like her just like that? Hope there's some background story for this cus the affection is pretty weird