By describing in unappealing and excessive detail the process by which humans speak, the kid on the left has weirded out the kid on the right, who now finds it too weird to talk normally -- or perhaps, by being forced to think about something that otherwise comes naturally, he has lost the ability to actually to it, could go either way.
The kid on the right communicates his distress, and indicates that it's the left kid's fault, by writing a note to the left kid since he can't speak the words out loud.
Oh. Thank you kind stranger. For some reason it didn’t click that he was passing a note. I’m for some reason I thought he was writing the whole time and friend left just noticed or something. I missed the obvious eye contact in the first two panels cuz I have no social skills.