It's not him only ever clearing obstacles, he reinforces self-control, consideration, empathy, and the innate positive traits Diana already has. He is still a siscon, still caught up in his own obsession to clear the future, and has so many weaknesses and Diana gets to be her own active agent in affecting Cain (like she's so precious and the message that children are a bundle of potential and possibilities is so heartwarming).
Besides, teaching her to show a facade fitd the nobility status (and a lot of the parent nobles in most fantasy stories...esp the "elegant and wise wife being secretly as skilled or even better that knights or were previously adventurers before being a noble wife"). Got me so excited that I was stunned by the "recommendation page".