When people say the prince wasn't a normal kid in the beginning of the story they were not kidding. Cil being super smart for his age wasn't some manga hijinks it was actually a sort of condition that runs in the imperial family. Once every few generations a royal will be born with these incredible abilities but the consequence of this intelligence is that they will be emotionless/hollow unless they find something that makes them feel fulfilled. In the past, some don't and just become hermits, others found solace in some study or in conquest, and in some cases just being in love was enough to fill that void within them. And we learn the King hoped Cil would find love, by having the Cil engaged to the eccentric Bertia.