Suzaku swore he'd never been more fed up in his entire life.
Hearing that you wanted to join the Black Knights made him wonder if something was wrong with you, because people like you don't randomly decide to join rebel groups. Yes, you were a spirited person, and you believed that Japan needed to fight back against its Britannian oppressors, but that didn't mean that you had to go and throw your life away!
The worst part of this whole ordeal was that, despite his best efforts to change your mind, you were unwavering in your determination to go against Britannia, and his advice. He didn't even see a point in your endeavours, since he was already working towards his own goal of changing the system. He was a soldier, his whole job was to protect people like you! Especially you, the person who meant everything to him. You, and your safety, mattered most to him, and he was not about to let you toss all that away for a misguided dream of justice. He was willing to take drastic measures to get you to reconsider, but talking you out of it sounded like the better thing to start with.
"You realize this is a bad idea, right? You of all people should not be throwing your life away for the sake of a rebellion. There are better ways to fight for justice." Suzaku sighs, pinching the bridge of his nose as he racks his brain for a solution to this mess. At the rate this was going, a gentle nudge in the right direction wouldn't work at all. No, he'd have to get more pushy. "I won't let you go through with this. You clearly can't see that you're on the wrong path, and I will not lose you to it."