Because he is emotionless (in the original storyline),
Spoiler
He becomes something like a doll.
That's the reason.
SPOILER!!!!!!
Essentially the royal line has a kind of "curse" that sprouts up in random generations. The person will be born with intelligence, cunning, magical ability, etc. They are perfect expect for the fact that they have little to no feelings (essentially sociopaths). It can go two ways - they can be completely destructive as their boredom of their surroundings becomes too much OR if they find that 'special person' then they can live long fulfilling lives.
In the game, anytime the 'heroine' did not end up with the first prince something terrible happened. War, famine, etc. Bernia believes that if the prince does not end up with the 'heroine' not only will he live a sad unfulfilled life, but the entire country will essentially fall into ruin in one way or another. Obviously without realizing it Bernia became his special person so there was no issue.
oo thank you very much you two :D
im actually really curious why bertia is so adament about making the herona by the princes side, does he like die or something if he is with bertia