He didn't want to admit to it. He didn't want to admit just how much Matthias consumes his thoughts and how it gets in the way of the rest of his life. It's not quite reached the point of being absolutely detrimental yet, but it could very easily get there. Yet it's not his life he's worried about so much as Matthias leaving him. He worries that Matthias will come to realize that it's weird or creepy or something that Seong got so wrapped up in him so quickly and decide he's done. He's worried that this is doomed to fail now because of him.

Seong moves to sit sideways on the couch, turning his entire body toward Matthias. "Ten percent." He would've agreed to more.