Kai hadn't really thought about it. BloodBound was like an open wound for him, the red sun a bright beacon in his nightmares. As he heard Philip tell him he would leave he realized this city was not the one he should be anymore.
Distance, that's what they needed from each other and possibly the thought of never seeing each other again. There was no city though, no country that wouldn't tempt the lovestruck prince to simply go out into every forest and search for him at some point.
That's why he believed he should imprison himself. Put a stop to the madness he called his love and return back to where he came from, hide behind the pretty tapestries and the heavy curtains. His room, the soft sheets and the people all around him would help him forget.