Jon's shock in panel 2 must be the realization that Garfield can swallow his head. Or perhaps it is envy that Garfield can yawn with an exclamation point.
Now, I realize that Jon is really supposed to be an all-purpose "loser," but sometimes that means he's boring, but sometimes means he is eccentric to the point of possible mental illness. All we ask is a little consistency, Garfield! I'm not sure what I think of Jon's increasingly frequent habit of sucking in his bottom lip to further accentuate his overbite when he is irritated with Garfield's behavior. I am sure that I like that today's strip is about a man pissed off that his cat doesn't want to hear what he thinks.
The bonus joke here is that Jon also "knows" what Garfield thinks, on an almost daily basis.
Lordy, I've been looking at Jon calling women for so long that the table looks barren without the phone sitting on it.