Any casual acquaintanceship with Garfield will tell you that the cat spends heroic amounts of time sleeping, eating, watching TV and lazing because he loves doing these things, and hates denying himself anything. Too true, but unlike his cartoon glutton layabout forbearers like Jughead Jones, Shaggy Rogers, or Dagwood Bumstead, Garfield is not motivated solely by joie de vivre. A true divergent thinker, he also tends to take up an oppositional stance for the purpose of flustering others, bucking expectations, and making excuses to himself for his actions. In concrete terms, Garfield not only stays in bed all day because he is lazy/ has nothing to do, but because he knows it bothers Jon. Why it bothers Jon is another set of issues. For the most part we can assume that he sees in Garfield's refusal to get out of bed the futility of his daily life, an acceptance of laissez faire "why bother?" nihilism that Jon is not prepared to make.
Who knows what Jon is really up to, but Garfield is so entrenched in habits of manipulation that he can only assume that acceptance and support is the kind of scam he's used to pulling himself. The panic in Garfield's eyes when Jon seems to approve of the cat's plan to stay a-bed all day is that of a master reverse psychologist faced with his own tactic.