What's up with this guy's smile?
Why does it produce so many conflicting emotions in Kyon?

The Melancholy:
>After Koizumi left with that annoying grin of his, I discovered Nagato had left already.
The Sigh:
>I was speechless for a moment, and only came to my senses when I heard someone giggling besides me. That could only be Koizumi.
>“Oh, I see.”
>He seemed rather pleased; I really am envious of him.
>“How I should say this? As expected of Suzumiya-san perhaps? Only Suzumiya-san could have come up with such characters, simply amazing.”
>Don't smile at me like that, you're making me uncomfortable.
The Boredom:
>I glared at Koizumi's increasingly irritating smile. This was the big announcement you were talking about?
...
The Dissociation:
>Next to me was Itsuki Koizumi, smiling like always. Unlike the airplane contrail I'd been looking at, his face was neither pleasant nor unpleasant, merely normal, and there was nothing to be gained by gazing at it. I looked forward once again.
...
The Surprise Part One:
>Koizumi's light smile was like a serene May sky after days of continuous rain.