From the view of the character:
It is because Soo Won was raised like this, in boxes. The way he addresses his father and I guess even his mother makes me believe that Soo Won didn´t know any other way for the first part of his life.
It was just after knowing Hak and later Ogi and his friends, that Soo Won must have known informal speech. That is the in story theory.
From the author´s point of view:
But I also believe that the formal speech always signifies distance that
a) Soo Won knowingly uses formal speech and is known for this as he literally puts a distance between the real him and his role as a king
b) but also between himself and and everybody around him.
It is demonstrated time and time again, that Soo Won surpresses his real feelings for his role as the king...
However...he cannot maintain that mask for eternity and he breaks down in certain instances...then he uses informal speech in order to signify that this is not a role and it is nearer to his realer feelings.
Soo Won is greatly agitated,when he learns that Hak is becoming a Sky Tribe Soldier...Hak had always maintained a distance in his former life as a bodyguard...and now he suddenly becomes a part of Soo Won´s tribe...He approached Soo Won and I guess Soo Won is crazily happy.
It is just that Soo Won hardly shows really strong reactions. When he does, he often uses informal speech
like when he was shocked, that Hak became a Sky Tribe soldier
or when he was screaming at Yona...He was agitated, but also made sure- but mostly unvoluntarily that he meant Yona and not Hiryuu...Soo Won shows his own rawer emotions.
or when he told Hak, that he is selfish...expressing his pain and his feelings of guilt.
Soo Won´s persona and formal/informal speech
The role Soo Won is playing is so ingrained in him by his environment that he hardly can actually escape it.
This role is signified by his formal speech or his way hardly to express strong emotions, to be logical...the ruler or leader...
But I would not see it as a carmouflage...that is just a part of Soo Won...and the other one is the needier person...the one who wishes for friendship and love and acceptance...
Whenever his more vulnerable side is shown, he uses informal speech to signify that there is less distance between him and the other person, while he usually keeps a distance with his formal speech from other people.
Hak and Lily and formal speech
Soo Won is peculiar sensitive to formal speech...when people try to put distance between them and himself.
Sometimes he wishes to be nearer to these people: Hak and Lily.
Hak did put a barrier between him and Yona, but also between him and Soo Won...calling him Soo Won-sama or Lord Soo Won. That might have hurt Soo Won, therefore Hak should just call him "Soo Won" like he used to be...
Or in Lily´s case, Soo Won tells Lily a couple of times, that he "doesn´t mind if she uses informal speech"...actually he expresses in his own way, that he prefers it, if she uses it...
if she does not only look at him as king, but as friend...After all, after they return from Ogi´s place he told her more or less, that he wishes to be her friend...
Yes, Soo Won most likely will use more often informal speech with people he cares about and open up more, but I guess he will still continue to use formal speech in his every day life.
snowghoul
turnnoffyourmind
romaincrisis
