Jon has begun unconsciously (?) imitating one of Garfield's signature leaning-on-the-table poses. I hope this slowly creeping hero-worship begins manifesting as Jon walking barefoot on the kitchen table, kicking Odie over the side, and sleeping in a box with a blanket over his head. The poignant thing about Jon's delusion is that he seems to know his life isn't currently amounting to anything, but he will succeed in this world at some undetermined point in the hazy future. You call this hope?
It's fine that Garfield thinks Jon's utter failure in all aspects of life is hilarious, but his expression in panel two is what makes the joke. The orange-boy is surprised by his own reaction. Can Garfield not believe that even he could be so mean? Is he aghast that his own body is in revolt? That his autonomic Schadenfreude-response has revealed to Jon the black-striped depths of the feline soul? I guess he's also in pain. Anyway, that expression is the Panic of the Busted. Jon's response indicates he knows perfectly well what that "SNORT!" was. And Garfield is busted.
Pointless admiring observation:: Garfield in panel three with his eyes squinched shut and ears flattened is an adorable and rare display of kitty behavior.