Every reader understands the basic gag today, and that it is accurately portrayed cat activity to get up in your face, disrupt what you're doing, and sit on your reading material when they want something. Like it or hate it, the self-centered rules of the cat world can be a funny, blunt mirror of how people treat each other. It's an especially nice touch the way they purr and try to look cute as if you should appreciate their demands for attention.
There are subtle shades to Garfield's particular brand of amalgamation of cat behavior and human behavior. Garfield, perfectly capable of getting his own food, likes being a bother. Likes it very much. Garfield's lazy desire to be waited upon, and the way he sort of exploits the situation to provide an excuse to be rude are distinctly human. While he may not look happy about what's going down, Garfield does throw the audience the loaded glance that means he's about to roll up his sleeves and get to work. I would not hazard to say that Garfield would enjoy being hungry for the mere chance to pester Jon (some priorities outweigh all other considerations), but when opportunity pops its head, the man-cat pounces.
Meanwhile, panel three provides an unfortunate reminder of Garfield's salami-thick tail.