So... what if there comes a point in Clarus' life where he says enough is enough and "quits" being the king's Sworn Shield? Maybe on their one on one meeting, Regis is quite adamant about some plan and refuses to change his mind about it. Clarus hates it and says something along the lines of "ok, I'm done. Screw it. Why don't you ask the Marshal instead, hmph"

Or he's just jealous of Cor because of so and so...This was better in my head sadly but yeah.

Bonus points: Regis apologizes to Clarus and they have the most amazing makeup sex