Garfield frequently takes pleasure, brags about, and shows off his bad behavior. Today he is amazed and held in thrall by the power of negative feelings to overwhelm and transform him. Similar to a recent gag in which Garfield finds amusement in his own boredom. To a point, Garfield always likes to wallow in negative emotions, which is why he insults those he cares about and frightens and hurts the innocent. But those are behaviors that allow him to victimize others to make himself feel powerful. There is a certain usually unspoken thrill in the feeling of giving way to damaging emotion (the comfort of feeling sorry for oneself, the adrenal rush of anger). To be sure, Garfield is feeling this in panel one. But by the end, where the cat is delighted by the physical transformation caused by grumpiness, and fascinated by his apparently unmotivated mood swings, he's degenerated into pure naval-gazing. He just finds everything he feels and does endlessly interesting, and wants to tell others about it. This is a testament to Garfield's dedicated brand of narcissism.
Panel Three: all tied up in thinking about his own mood, Garfield attempts to summon Jon from another room by "shouting" - i.e. thinking loudly at him.