Shun sees through this and turns it around, he's basically going 'Stop tryharding the big sis role, you're completely wrong about me, I'm no more happy playing than when I'm spending time with you. On the other hand I know all about you, you're gonna feel lonely without me'. That was 100% intentional and confident, he knows what he's doing, and he got the point across. Naoki's flustered because she realized he's spot on, she's attracted to him and Shun more or less knows.
This isn't actually what I was referring to. Nor what I believe the original commenter was referring to.
Yes, he is clearly confident in his understanding of her as a person, but his brief moment of annoyance, then his building up to and then not being able to commit to what he actually wanted to say, both contribute to the moment not feeling "smooth" to me at all. Which I think is in character.
He did express himself clearly, but subtextually, he had something else to tell her that he wasn't confident enough to say outright, and he couldn't keep his emotions in check well enough to calmly reassure her.
Both of those things would take confidence in himself and their relationship that he doesn't have. Not confidence in his understanding of her, which is what he displays