Basically the royal family, starting with the first king, occasionally have someone who's a born prodigy.
But they have one flaw, in that everything just seem so boring to them.
This state will last until they either die out of boredom, or they find 'the one thing' that pique their interest.
From then on, they'll basically be obsessed with that one thing.
In case of the first king, his one thing was his childhood friend, who basically acted as a moral leash to keep him from going too far, and thus the country prosper under his rule.
In case of Cecil, Heronia 'can' be one of the ones to trigger this, hence the idea that she's the 'fated girl'
(if it's not obvious yet, Bertia already achieved that trigger since their first meeting)
Btw, there's actually other ending for Cecil if Heronia doesn't go after him in the game, where he find something else to enjoy.
Ranging from:
Research - he become a shut-in researcher.
War - he become a tyrant who unite the whole continent, and keep going because he need more war (this is why Bertia panic'd when he started showing interest in gunpowder)
And I believe him never finding anything is also an option (thus the 'android prince', because he's still apathetic)
This is why Bertia is so insistent on pushing him onto Heronia, she think that right now he's just acting the part, and inside he feel nothing, that's why she want him to find the 'fated girl' and finally get to experience real emotions.
Because at her core Bertia is a 'nice girl', so even if it means her own ruin, she just want him to be happy.