Jon is suddenly unexcited about going to the vet. Perhaps he realized he hasn't time to grow a mustache, or that it's pointless to fret over what to wear when you only have one shirt. This lack of enthusiasm is a strange, unexplained twist, but at least it makes the formality of his phrasing "we must go to the vet" even funnier.
Garfield is just trying to voice his hatred of getting shots, I know, but his grumpy complaint seems to imply: A) Dr. Liz thinks hypodermic needles - probably several - should be stored in pincushions, or B) when Liz gives cats a shot she simply jabs the needle in and leaves it there.
The "camera" tilt-up to accommodate the thought bubble of standing Garfield in panel three is distracting, but not as distracting as watching Garfield's hind legs go from thick and blobby to scrawny. The pincushion is ready to have confusing anatomy when you are! It is cute that Garfield is still excited about cat treats, when he can eat entire cakes, pies, and platters of cookies at home all the time.